Combat rating starts at a base value of 100 for everyone, if you play 'within expectations' it doesn't change much, but if you do better or worse than those expectations it will change alot. I think it takes the teams average combat rating, and which ever is higher is expected to win. I don't know if it accounts for individual kill/death in the same manner or not. If a team has a much higher rating than yours and you lose, you don't lose much, but if you win you gain alot. The opposite is also true, if your team is much higher and you win you don't gain much, but if you lose than your rating is dropped alot. I don't know how much more goes into it, someone else can correct me if I'm wrong.