I've had similar stuff happen and I can't figure out why it happens. I think it has something to do with your connection to Xbox Live. If I'm playing single player, I load the leaderboards up after every couple of games. That seems to keep me from losing any of my single player scratch. If I'm playing multiplayer and I win, I never choose Rematch. I always exit back to the main menu. After a couple of secondsthe scratch updates. Then I head back to the multiplayer lobby.
Your scratch will definitely *not* save if you do not connect to Xbox Live. I unfortunately found this out after an airplane trip where I disconnected, accumulated about 30k in scratch on the plane, and then realized later that none of it was saved.
My leaderboard entry shows I have around 500k. My in game total in the top right is around 400k. And my total from summing all my stats is around 480k. Go figure.