Flamestrike
Legend
Skill and luck. The reason the character isn't killed in a single blow is because the attack is more or less evaded. It would have killed the character (or at least left them dying) had it been a solid hit, but the loss of HP represents the character utilizing skill/luck to avoid those killing blows.
At least that's how I run it.
That's how its supposed to be run. Hit Points are defined in the game as 'Health, resolve, the will to live, and luck'
You get more of them as you level up in experience, moreso if you belong to a martial class that fights a lot, so they also represent skill and experience, and parrying/ dodging/ fighting ability.
When a 20th level fighter gets 'hit' by a Fire giants sword on an attack roll, the blow actually luckily misses him, thundering into the ground next to him, as he twists out of the way, and readies for a devastating counter attack.
When the same blow 'hits' a 1st level Fighter, the poor schmuck is cloven in half.
Its the redshirt/ Kirk effect.