I'm not 100% sure how it is with ghosts, but in mw2 xp lobbies have nothing to do with score or leaderboards and I'd say that they do not affect the leaderboard in ghosts at all as CoD handles leaderboard/score separately. It all goes down to the server you were in, but with that said most of them are just using scr_xpscale that does not alter the score at all.