It's called “Hit points”…and it's not so much a hidden mechanic than a logical concept. Ogres are big, so they can take more damage. Keepers are small, so they can take less. That's why they have more tricks up their sleeve to avoid damage, such as invisibility, illusions and swarms of flies. They're not your average RL warrior which can just hold a directional key and kill everything in its path…you have to think every move.