In Black Ops Nazi Zombies the leaderboards reset my top round.?

I went away for a little, and came back to my game. I then looked at the leaderboards, only to see that my top round of 25 reset to 6. I've gotten past round 6 thousands of times. I've compared it with my friends, and only mine changed. This is on kino der toten. I've also shut off my xbox and turned it back on.