The thing is, the AC concept as well as the whole combat system in D&D is abstract. Why would a high level fighter have 100 hit points? He is no more likely to avoid having his arm chopped off than a common peasant if the blow hits the right way. This is the "hero" factor, which is movies is why a rain of arrows fails to kill the hero, when others fall like flies around him. Of course it also reflects his skill, but it is a combination of skill and "hero luck".
For the armor class, you should understand that an "attack" is really an abstraction reflecting one round time worth of fighting, with several blows/swings/stabs involved. Maybe only one hits something other than armor, maybe the armor is hit and causes a bruise, many things can happen. Maybe the hit is just a scratch and doesn't cause real damage. The "attack", AC and "damage" are all abstracts. This is hard to forget, especially because animations suggests otherwise, but that is the combat system in D&D.
There are other systems out there where an attack is taken more literally and you would roll on tables to find out what happens in detail: A hell to role play, but it might work on a computer. It would not be D&D though.
So why does a succesful attack give the same damage to an armored guy as to an unarmed one? Well, statistically, over several rounds, the armored guy take less damage. Now you may like this abstraction or not, and I respect it if you don't, but personally I like it for its simplicity.
