Alright, I will preform another test when I can to ensure this is correct. If so, I'll get the moderators to lock this.
Don't forget it might be the hitboxes in garrysmod (aka, shit).
Most of that buck probably lands in the chest, not the head, since it spreads so much. That might be why they only get 80 of the damage, and not the full payload.
Try a direct on shot with someone in the T pose (or holstered, the TTT equivalent)
vs
Someone who has a gun and is leaning forward slightly.
Also take a shotgun and the pump at similar distances and shoot the wall. You'll se what I meant about the spread.