1. The Crysis 2 leaderboards simply go on who has the most of XYZ. This basically translates into who plays the most and its also the reason the same people will be at the top of most leaderboards at once.
2. You can progress on leaderboards while choosing your own opponents, playing in pre-mades vs randoms, hopping servers until you find bad low ranked players or avoiding good players and so on.
If you take the leaderboards seriously in Crysis 2 you are simply idiotic.