"Make up stuff in your own" isn't exactly a sign of good design.
Never said it was, but making up stuff with goalposts is a bit better than making up stuff without.
Prove it.
Ok, firstly, passing over 'boss' type creatures for the reason that they inherited the 'lets under CR this so its more like a boss fight'.
CR 5:
Wraith: Save based primary attack form, Con Drain DC 17, AC standard, hp slightly below standard but incorporeal so it halves incoming. Saves below standard, but undead immunities. Damage way below at 1d6, but Con Drain adds 5-15 more damage, giving approximately High damage.
Vodyanoi: Controller+gish type, SLAs DC 14(one shot)-12, AC above standard, attack bonus below standard, hp slightly below, saves above standard. Damage adds up to about 15 on a completely successful full attack, though it has shitty secondary attack bonuses so I doubt thats going to happen.
Nightmare: Brute type, smoke DC above standard, AC above standard, hp below standard, attack bonus standard. Damage low even on a successful full attack, but only bite has full to hit.
Kodama kami: Ambusher with SLAs, DC average, AC standard, hp below standard, attack bonus average, damage is average assuming sneak attack and doublehit, which is easier since both attacks are primary at least.
Gibbering Mouther: Freaky special attacker, DC high for spit but low for gibber. Hp is really low, AC is above, saves above, attack bonus low-ish, damage is 15 if all the attacks hit.
Megaraptor: Brute, hp slightly below, AC significantly(2 pts) above, saves are WAY up(on one save), attack bonus slightly below(but made up with having non-shitty bonuses on secondary attacks), average damage is a monstrous 43(considering a level 5 PC's hp, you either have enough AC to make 2 +9 attacks miss or you're dead), over double the standard. As a side note it has hilarious Stealth for a Large dino, and is likely to run in and eat someone before they know it.
Basidirond: Debilitating effect fountain. High DC, hp slightly below, standard AC, standard saves, high attack, below average damage(even including the spore con damage, its just barely there. On the other hand anyone who flubs the save is going to spend the first round tripping balls so its got time.
Basilisk: short range save or die machine, standard good DC, hp slightly below, below average AC, standard saves, standard high attack, laughable damage.
Ok, I guess I overstated the point, they're mostly only slightly squishier, and DCs aren't that low. Optimized players helps skew perceptions(I forget my crew is running around basically all the time with a Mass Conviction), the PCs kill everything more or less trivially prior to mods, and back up their defenses with spells.
Beyond that, you can see the damage values tend to be counted as consolidated offense totals, anything with a decent AC could trivially quarter most of them, except for the Megaraptor, who needs a decent AC just to survive being charged at all.
CR 10:
Tophet: Brute, standard AC, below standard hp(but DR), hilariously weak saves(but immunities), high attack(a whole 3 points above), good trample DC, middling average damage of 38
Siyokoy: Stun and Run, above average AC, above standard hp AND DR, no good saves, standard high attack(on multiples to boot), high save DC(3 higher than recommended), standard high average damage
Nyogoth Qlippoth: Controller brute(drop acid fog and wade in), standard AC, below average hp but DR to back it up, good saves with a weak point covered by immunity, average attack, but all primary, just below high DC, and low damage, even if everything hit. It'd be making that damage up with counters.
Pale Stranger: Ranged striker, slightly above average AC, slightly below average hp but with good DR to back it up, good saves and undead immunities, and a really terrible attack sequence because it uses iteratives rather than secondaries BUT hits for touch when in close combat. Damage is 6d8+6(avg 33, low) or 1d8+1+6d6(avg 26) or 4d8+4+6d6(avg43, high), depending on range and crittyness.
Nereid: Save or be screwed spammer. Below standard hp with DR backing, AC slightly above par, very good saves and SR too, with average accuracy touch attacks. DCs of 25-20 make it well ahead of the crowd. Literally no damage ability, it just makes you save or be fucked over and over.
Nue: Stealth Brute. Below standard hp, standard AC, standard saves, and just below high attack. DC go from 21(the poison) to 16-14(everything else), which is high for primary and secondary respectively. Damage is 40 before energy drain, which adds another 10 for stock standard damages.
Imentesh Protean: Multithreat. hp again below standard, with DR/fast healing supporting, slightly above par AC, with mostly good saves and slightly above average attacks. Save DC for its finisher(PAO) is significantly above primary standard at 23 but others are at 20 and below. Damage is swingy, with sneak attack, a large array of high accuracy natural weapons and constrict to mess it up. 33 base +56 sneak attack brings it to a staggering 89 average, twice that of the standard high. Good thing it has no stealth abilities other than shapeshifting eh.
Guardian Naga: Caster. Hp is significantly below standard, AC and saves mainly standard, DCs of 21(poison) to 17(spell), making it high but normal. Damage of 9d6(31 avg, spell), 24(bite + poison), makes it not all that damaging, but it has invisibility and divine power to use an attrition strategy.
Zuishin Kami: standard hp with fast healing, below standard AC, good saves, standard primary DC for Dispel Evil, and a good, but iterative based attack bonus. Damage is average(38) against non-evil, but the Holy adds another 14 to make it high against evil, assuming Rapid Shot is used.
Gug: Brute. standard hp, standard AC, slightly below average saves, and good attack bonus tradeable for damage. Damage 53+30(power attack), which is either High with high attack or very high with low attack.
Fire Giant: Brute. Above standard hp(how rare), standard AC, regular saves with a critical hole in the Dex, above high initial attack, but iteratives drop the latter attacks below Low unless using the low damage Slams. Damage(with Power Attack), of (3d6+24)*3 for 82 average on weapon(risking misses on the last two attacks), also double that of High, or the average but more accurate slams of 41 combined.
Giant Flytrap: Ambusher(mostly because its barely mobile at all), above standard hp, below standard AC, similar to the Fire Giant, it has one good save, one ok save and one useless save, but plant immunities, low attack bonus as well. Damage wise, while it has Vital Strike its not all good for a multiattacker, and its accuracy is too bad to Power Attack, 46 average for all hitting, which is standard high(but with its accuracy, it'd be lucky to land half of those)
Brachiosaurus: Big ass brute, enormous hp for the level, but AC like the broad side of a barn, saves are good(except for Fort which is wtfgood), attack is well above average, but only a single tail whip. Damage averages 33, which is low, but Power Attack turns it to 45. Maybe if you Cleave the whole party with the tail sweep.
Piscodaemon: Magical Brute(debuffs and beatdowns). High hp for level AND DR to boot, standard AC, standard saves, as well as high to-hit. DC too is higher than standard on the poison, but strictly secondary grade for the SLAs. Average Damage is 53(inclusive Bleed and Poison), which is fairly standard. Watch out for later rounds though, what with the bleeding and poison.
9 headed Cryohydra: Melee Striker. Absolutely atrocious hp, even worse AC, but fairly normal saves. Its attack bonus is below Low, but the breath DC is high. Average melee damage is 76, average breath damage is 95, if it can get to melee since its Huge, with no stealth ability and slow at 20ft. Maybe if you ran into it in a closet.
Couatl: Sneaky caster. Standard hp, below standard AC, good saves, average attack(not that it needs to use that). SLAs, poison and spells DCs range from 20(plane shift)-15, which is around Good. Damage is well below par, but this is a pure caster foe with at will Invisibility/Plane Shift and Charm Monster DC 17 4/day. It does not need damage.
Bogeyman: Magical Ambusher(duh). About as squishy as the hydra(it has DR 15 though), with slightly below standard AC, good saves. Save DCs are high, which is bad, since it has Phantasmal Killer at DC 21 and its claws have a DC of 25. Attack bonus is low. Damage, if it goes melee, is 53, which is standard high damage. Its real danger is all the SoLs it'd be throwing around, with its fear aura and SLAs.
Bebilith: Melee brute. High hp, lowish AC, standard saves, high to-hit and a nasty DC 23 rot. Damage 44, which is lower than standard, but Power Attack adds 12 to make it a respectable 56 and Rot ramps it up further to 66 average. Nasty customer.
Movanic Deva: Caster/Tank. Standard hp, AC, saves, attack(but iterative based), as well as above average DCs. Average damage of 52, goes to 88 with power attack(but probably missing the latter iteratives entirely), alternatively Vital Striked Power Attack goes for 36(Cleaved to double that), which is higher than non-pouncers can mange on the opening round. Or if facing evil it would just fly and Holy Smite your face to bits.
At this point, it seems hp is consistently below the bestiary recommendations, but many cases have DR or some other damage mitigation. The damage recommendations seem to be based off the assumption that all attacks hit and that they get full attacks, which depending on their secondary attack bonus, may not be a given. Power Attack does not appear that frequently, however appropriate it might be, likely because it'd drive the damage values up significantly. Critical hits are not accounted for.
Numbers in general adhere closely to the given statistics on published creatures, generally deviating by +1-5(gap gets larger at higher CR) on the d20 numbers, and larger variances on hp and damage. A number of creatures however, have more extreme outliers, and deviations are generally higher than the baseline, except for hp, and iterative based to-hit. Damage is surprisingly consistent between the goalposts, except for particularly hard hitting creatures with weak secondary attacks, those get to almost exactly double the cap, personally I suspect the designer for those creatures counted damage after accuracy is accounted for, rather than before. For a given creature, 2-3 stats would be above the standard, and 1-2(depending on how you count saves below. Additionally, damage estimations seems to only consider the first round of damage over time, as the cumulative Con loss from an effective poison may be disproportionate over time. Relatively few things have save spamming, they tend to have the bulk of saving throw effects behind a standard action, or else tied to only one natural weapon.
Those numbers also make monsters defensively rather weak, a Fighter with the weapon training bonus can power attack all the time and hit pretty much all the time. Output wise, barring crits and extreme damage whores, the majority of monsters take two rounds to kill pretty much any PC(minimum of HDd6+3*HD(Con 14+Favored Class) makes it 65 hp on a squishy wizard at level 10, while the monsters hit for just above half that) given even favorable situations like all full attacks connecting and fulfilling all their extra damage conditions. Having decent armor alone would shave off a third of this value, and good fort saves to resist poison takes off another quarter.
At level 5, minimal Dex(+1), full plate(+9) and a shield(+2) would get you an AC of 22, which is about 50% miss for their best attack(inclusive of high to-hit of +13), and significantly better for voiding secondary attacks. At level 10, adding another point of Dex, +1 to armor, shield and 3 magical AC sources at +1 gets you AC 28, which performs similarly.
