When the system is down it pretty much gathers all Game info such as Head shots, Kills, Assists, Score, Etc..
And if you have any unfinished games such as you backed out early and didn't finish the game it will erase those kills making it lower your level to whatever level you were before that game.
So pretty much if you had any Unfinished games you will lose all exp from those games.
WARNING: Don't back out you will lose all XP from the game and a chance at getting probation which doesn't allow you to play online for a certain amount of time.
Hope I helped everyone that had questions about this.