It's definately due to time differences, lag, and whatnot. It's been a long time since I've gotten blood decals to show up but have no damage dealt unless I die immediately after this occurs. (but man, was that aggravating when it I used to have to empty half a shotgun at point-blank to score).
I was over playing Halo 2 at a friend today, and while it handles lag much differently (worse IMO), the same principle works just fine: apply bullets until desired effects are achieved