Let me describe a scenario that I think we've all been in. You pick up a game like Gears of War 3 or Starcraft II or the deck-building iOS game Ascension.
You jam through the single-player campaign or do a little comp'-stomping in skirmish mode -- maybe even on the second-to-hardest difficulty 'cause you're totally hardcore like that. And you're better at the game than anyone on your friends list, judging by the local leaderboards and the way nobody will play with you anymore. You've got this game figured out, man, and you think you're pretty good.
So you decide to venture online and try your hand at ranked ladder matches, a tournament, or maybe even just some pickup games via online matchmaking. You get creamed. Murdered. Owned.
At the end of the match, your competition has left you with a kill/death ratio in a realm of negative numbers so low that mathematicians hadn't even bothered to think about it yet because they figured nobody would ever use them. This baffles you.