That's how this system works. You do not gain points continuously by playing longer, you gain points by being good at the game. You lose 5 to 25 points based on the point difference with your killer, and he gains those points (but if they're new on the server, they first have to get 50 points before they can obtain the full 25 points from you). Eventually you'll either strike a balance or you keep climbing the ladder because you get more than 25 points average per life (which most of the top-range folks do). Besides kills you also get points based on multi-kills, headshots, point captures, destroying buildings and other shenanigans. There are a lot of other options to keep your score high. The top ranked folk have a k/d ratio of around 2, so they are getting points through lots of other things.