Let's have a look at one of the simplest concepts of the game... from an in-character perspective.
Krogan the barbarian is obviously a well built and beefy guy. Krogan is injured easily, but also heals quickly.
Elijan the wizard is fit in some ways, but not in others, and certainly not very imposing. Elijan is difficult to injure, but takes more castings of the same spells to heal properly.
Tilek the healer has been volunteering at the infirmary for four years. He's seen almost every kind of injury, and had experience with treating things from cuts to bruises to burns to dragon bites.
Polam the cleric has dedicated thirty years of her life to providing healing in various forms to temples and the people of the city. She has been volunteering at the infirmary for a year and a half.
Krogan are Elijan are both delivered to the infirmary on the same day, and both are unconscious and in critical condition. Although stable for now, it's unlikely that they'll recover on their own. Tilek and Polam are both on staff, and get to their patients at about the same time. Seeing the terrible condition of these patients, Tilek and Polam determine (independently of each other) that mundane healing won't be effective enough to see these people out of the infirmary in less than a week. Since the beds will definitely be filled in fewer days than that, they decide to cast healing spells on their patients, and have them out of the infirmary within hours, at most.
Tilek casts a spell designed to heal light wounds on Krogan. Since he isn't being rushed, Tilek performs the castings slowly, ensuring that they provide at least moderate healing to Krogan. With two castings of this spell, Tilek has provided Krogan with a full recovery.
Polam casts the same spell on Elijan. Since she knows to take her time with a spell in order to ensure its effectiveness, she also performs the castings slowly. With five castings of this spell, Polam has provided Elijan with a full recovery.
Now then. There's more information to be had about all of these characters, but I'd rather not release it just yet. From what you can see, however, what are the most likely reasons for the above information to be true? That is, what statistical truths would you deduce about Elijan, Krogan, Polam and Tilek?
Furthermore, why does healing work this way in D&D?