Hitpoints

[GoC]

I'd go with the former, actually. Due to their better skill, they manage to decrease the effectiveness of the hit by pivoting so that it grazes them, parrying it (if a little too late), etc. I find it easier to visualize that way than an increase in actual tissue resilience (which would be what Natural Armor increases do).

I always thought natural armor was things like plates of bone or scales that made it harder to hit vital spots?