Who cares?
It's very simple - power ups are RANDOM and will drop at RANDOM but more likely when you kill lots of zombies at once (train) and lots of headshots! (or so I've noticed)
Now I hate when I play online and people PURPOSELY ignore the drop ups because they're deluded into thinking leaving it will mean more later. I had no ammo and someone was refusing to pick up a Max Ammo that was near them because of this BS..