Dawesome

So im the sneaky type, and i have the 15*dmg (Sneark) Perk so i can 1-shot things :P. But when i go behind a person it sometimes makes an animation cutting their throat off! which is awesome. But sometimes i just hit them once and they die without an animation (froim sneak etc) Anyone have any ideas if im doing something wrong ? or is it just at random this happens :) Cheers :)