It's possible that it balances the best players between teams in order to make the match more competitive, but then it's also possible that it uses an arbitrary magical number combination to divy them up. Most teams I play against seem fairly well balanced against the one I'm on though, so if I'm guessing the trueskill rating is somehow involved.
I think it has to do with trueskill more than anything. I'm not sure how the game determines which player gets put on which team but something is screwy. I haven't played Halo 4 enough to see the effects all the time but in Reach this was almost an every game occurrence for me. I'd have to carry players that were clearly not the same "rank" as me or the enemies I was playing against and it was something that made me stop playing Reach on more than one occasion.
"It takes an idiot to do cool things that's why it's cool"- Flcl GT - Hawk082
In my experience, only the enemy team is balanced with TrueSkill. My team? It gets balanced with MentallyDeficientSkill..
What this means is pretty much every game I will see their team stay in a squad of 4 and rain DMR/grenade death on everything, while I can sit and see my team mates HUD tags form a literal triangle across the map as they run like lemmings and die solo, one after another.
This is a really good question. From what I've read, Halo MM has been tracking our performace from day 1, and giving us invisible skill ranks. You have separate skill ranks for each gametype. These ranks will be made visible once 343 has gathered enough data and done some tweaking or whatever. Once thing this tells us is that K/D is not the only metric used in determining skill rank. My speculation is: Matchmaking pairs you up with the other players, whose skill rank is +/- yours by some predetermined margin. Creating a player base whose average skill is hopefully only a few points off from the best and worst players. Then it splits the base into 2 teams with that same average. In short: My theory is that MM doesn't balance teams based on best players, it creates 2 teams with the same average skill rank.
If I had to guess I'd think K/D, win %, and average score are the main factors. I also feel like its adjusting game to game. For example I'll have a really bad game and next match it feels like I can't lose, then another where things feel even between teams, then a third where it's a struggle to stay alive for 30 seconds, and so on.