Assumption 1: Human stats vary naturally between 3 and 18, weighed as a standard 3d6 bell curve
Assumption 2: Humans in our world do not go beyond level 6 (go dig up various articles debating that point yourself)
The highest mental stat a person can have in our world is thus 22, but since it increased with age, it can't correspond directly with IQ (as IQ is meant to be the same over one's lifespan), so we are disregarding Int gains from aging.
The highest IQ that has been measured is 228, according to my (admittedly short) research, and one standard deviation from average is 15 points, so this corresponds to 8.5 standard deviations. What this leaves us with is this: the bell curve for rolling 3d6 does not at all match up with the bell curve for IQ.