this is due to client-based positioning I believe.
Many times I stun a player in the fire, but he takes no dmg and when the stun wears off he's magically standing 2m behind it.
Same goes for knockback, just when you think you're gonna throw someone into a pit of doom, he goes 45 degree some-other-way and survives.
I think that the player is not in the same position on your screen as he is on his own, and that lag causes this.
Happens A LOT and I've got 100mbit so it sure as hell ain't my interwebs.