Alright, let's say that in some cases, a hit from a giant's club is a glancing blow or the character bruises himself hitting the ground after being nicked and diving away. The point is that in this system, it is impossible for a giant's club to ever deal 'true' damage. What, he never lands a solid blow? That doesn't make sense. On at least some weapon hits that I can imagine, damage is 'unavoidable'. How are those attacks to be handled?
Sure he lands a solid blow: whenever the creature can't deflect or absorb more hits. For a first level PC or a commoner, that'll be almost immediately, because 1 hitdie is very little. A higher level PC with more training can take a little more: it's going to take a few solid strike againsts the fighter with his d12 hitdie, or just one vs. the wizard with his d4 hitdie.
Really, this is no different from normal hitpoints: Why don't the first several inflict real damage? Why is that sword cut healing overnight? Because hitpoints don't represent wounds, they represent the ability to
avoid them (at least, that't the interpretation I think jives best with the mechanics). The first real swordcut is the one bringing you to 0 hitpoints, which (reasonably enough) instantly drops you and causes you to slowly die by bleeding out.
All a wounds system introduces is a bit of a broader margin between the "just mildly battered, nothing a nights sleep won't cure" state and the "festering stomach wound that incapaciates you and will likely kill you if left untreated" state.
The reason you want hitpoints as-is not to represent real wounds is that a real wound is generally crippling, and having it depend on just a single die-roll---the attack roll---makes combat so lethal it must be short or you'll be losing PC's every session. And even then, an unlucky initiative roll will inevitably mean a high likelihood of a crippling wound. Therefore, exciting combat with any amount of give and take
necessarily implies that a single attack roll cannot by itself cause serious wounds to make the game playable.