When the ladder updates, it re-calculates based on every game that finished, not just your most recent ones. Just because you finished a game and your rating change, it doesn't mean that game was the only thing that changed your rating.
I've never seen a case where a win causes a point loss. Every time someone thinks that, the end result is actually something else that caused the point loss (someone else's game finishing or a game expiring.)
You can prove this yourself by following the instructions at http://wiki.warlight.net/index.php/Ladder_Ranks_and_Ratings
. You can see what your rating would be if you hadn't played that game at all.