It changes based on numbers of people on the teams. It usually moves someone with a mid level score to the other team.
Teamwork is key on these servers. When people are not working as a team I usually go out and pwnsauce it up, because everyone wants to cowboy up and usually go alone or in pairs. If that is the case I usualy switch teams mid-stream and try and help the other team win. When teams work together, things usually go pretty smooth for both sides and my scores drops quite a bit because I die alot more often as does everyone else. When teams work together, kills are usually pretty evenly distributed and the people on top will die more often.
Also, keep in mind that if one team is working together and the other is not, then the team working together will almost always win regardless of skills. Team stacking is often confused with teamwork.
EDIT: And I almost forgot. Welcom to the forums. Glad to see some more new faces here.