So, if the 100hp dude isn't actually capable of withstanding significantly more physical injury than the 20hp dude, but is instead good at avoiding the full force of the blow, then presumably he's not getting as physically injured. Yet it's perfectly possible that he requires a lot more healing than someone with lower hit points, after taking less actual damage. Why? What privileges one particular hit point over another particular hit point?
You know, I am absolutely sure that I already answered that.
But, I will do so again:
My full health (in hit point terms) is not the same as the full health of the prizefighter (in hit point terms).
When I am at full health, and he is at -80 hit points, we might have exactly the same physical injuries remaining. The only difference is that, because he is competent, those injuries hamper his ability, while they do not hamper my incompetence. But (and this is important) even hampered, he is still more competent that I.
0 hp damage =/= completely uninjured. Injuries can (and do) fall below the threshold of 1 hp in D&D.
If this still seems odd to you, imagine any pro athelete on his best day, and on a poor day. Full hit points in D&D, as hit points accumulate, begin more and more to represent that best day. Little things can throw the athelete off his performance, which you and I would hardly notice. Not because we are better, but because we are far, far worse. You will note that there is a real difference between that pro on his best and worst day.
Imagine, instead, that it is I on my best and worst day. Far less difference. In fact, you might not even notice. Low level hit points are like that.
Yet, even at only 20% of his best, the pro is at least twice as good as I. And, at 20% of his best, the pro might have fewer actual injuries than I do at full hit points.
Or, to (re)quote Gary:
Consider a character who is a 10th level fighter with an 18 constitution. This character would have an average of 5 1/2 hit points per die, plus a constitution bonus of 4 hit points per level, or 95 hit points! Each hit scored upon the character does only a small amount of actual physical harm -- the sword thrust that would have run a 1st level fighter through the heart merely grazes the character due to the fighter's exceptional skill, luck, and sixth sense ability which caused movement to avoid the attack at just the right moment. However, having sustained 40 or 50 hit points of damage, our lordly fighter will be covered with a number of nicks, scratches, cuts and bruises. It will require a long period of rest and recuperation to regain the physical and metaphysical peak of 95 hit points.
RC