Why do teams suck on black ops?

Yeah so everytime i join a game of demolition i ALWAYS end up on the sucky team. im not even kidding i never get the good team. my team always has the guy that goes 1-20 and the other team has two to three people going 57-6 who us! but I do pretty good against them, like 22-18. if i join in the middle of a game the enemy always has a chopper gunner out or attack dogs. Is it like this only for me or is this for everyone? i think its a bunch of bullshit that the teams are so unbalanced.