Per ENEMY means, if you stab a biker with a lot of (max) health, your chance per HIT is lower than when stabbing a weaker zombie, say, the cheerleader. The chance for dismemberment per ENEMY is the chance given at the weapon's respective Attribute. (I'll stick to knives as example in honor of the thread.
Since we've already derailed this thread completely, I'll try to sum up my current understanding.
