Even if you are atop the leaderboards doesn't mean you're the best. They have just played soooo much that they can rack up their score. I know WorldWar3 had around 75 days played and was number 60 on the score leaderboards but had a 0.8 kill to death ratio. Half of them probably don't play for real and just do domination headshots. The 20,000 people that are really good at COD4 could all be about equal. There are no large leaps of skill from the 20,000 ranked player and the number 1 player.