it's actually stupidly easy to get the number one spot even if you join late in the game. i've played the game like twice and i've been number 1 at some time in both those games. i'm not sure why that is.
maybe it's because the game is full of invalids
maybe it's because it only counts single kills and doesn't take into account the fact that kills are harder when you're at the capital ship level, so killing a fighter is worth as much as killing a capital ship.