[Official TF2 Blog] And that's what ye git fer toochin that!

Robin Walker of VALVe writes:

"One of the things we've been thinking about for a while now is how to improve the player experience around finding a server to play on. It's a tricky problem because our master servers need to ask a game server for its details, and that server can lie to us if it wants to. We decided we needed to find a way of scoring servers, with a goal of finding and delisting ones we considered "bad". The scoring system had to penalize lying without penalizing custom game rules, because some players like custom game rules. Best case, the system needed to work entirely from data that didn't come from the servers themselves, so they couldn't lie to us in any way to affect it."