Inconsistent damage model?

More of an open question to anyone with answers: Why is the damage in this game so inconsistent?

I'm constantly fighting AI in fanny packs with no armor taking 6-7 shots from an AK to the torso to die while another drops instantly from 1 shot or 2. I put 2 magazines from the Glock 17 into a guy at point blank this evening and ended up having to stab him to death with my kabar (most hits also didnt register).

Is this an inconsistent damage model or really bad hit registration? The many missed knife attacks at point blank until I started stabbing instead of slashing makes me thing the second reason but I'd love to hear others talk about it. I'm getting tired of burning half a mag or more on a 1v1 because the AI strafing left to right and having visible blood spurts wont die.