Since we've already derailed this thread completely, I'll try to sum up my current understanding. (I'll stick to knives as example in honor of the thread...

)
The chance for dismemberment per ENEMY is the chance given at the weapon's respective Attribute. AGI for knives, PER for rifles etc. AGI 1/10 gives 5%, 10/10 gives 50%.
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. It'll take more hits to kill the biker, so you get more chances, so each individual chance has to be lowered to reach the same chances per enemy.
How is the chance per swing calculated? I don't know, exactly. The numbers we've come up with in testing indicate it is roughly
( damage_per_normal_hit / Zombie_max_health ) * Chance_gained_from_AGI.
(ex. 200 health on a zombie, 40 damage on the machete, 50% from 10 AGI =>40 / 200 * 0.50 = 0.10 = 10% per hit. Even if you hit the head, doing triple damage, it's still 10%)
It may well be something more mathematically correct (that formula won't land on the exact given percentage), but that conveys the intuitive idea.
Note that since this relies on damage_per_normal_hit, the chance per hit improves with machetes over hunting knives (over bone knives). Sledges doing the most damage per hit, will have the best chance per swing (given equal values of relevant Attributes). This will FEEL like a better chance in game play.
Each zombie has it's own individual resistance, which will lower the chance some.
That's about all I'm pretty confident about right now, plenty of things to for me figure out still:
- why was I getting a ton of dismemberments on a magnum with 1/10 AGI - do weapons still have their individual chances as well, maybe?
- does armor play a role - you can reduce enemy armor by several means, will that effect your chances?
- is it decided by: base weapon damage, +mods, +skills, +buffs, +whatever else
- etc...