A question on matchmaking.

So, this has actually bothered me for quite a bit and I hope to find an answer here. My question is of course based on match making, more specifically, the match making of normal games. Is it on wins? # of games? Or is just kind of loose by lv 30?

My account is level 30 with about 165 wins to 145 losses in 5v5. This means I've only played around 300 normal games in 5v5. Notice how my score is neither very impressive nor that much experience in the game when compared to others. What I ask is why I get paired with many people having 1000 wins and occasionally people with 2000 wins somewhat often. It's not every game, but it's often enough to cause some concern. Going by the assumption that these people have won 50% of their games, I'm placed against those who have won anywhere around 2000-4000 games, and at times I wonder why.

I tend to play with friends, but the highest amount of wins of them is only about the mid 300's, giving them around 700 games. Anyway, this is getting long so I'll try to rap up by asking the same question as I started with. How is match making in normals decided? Is it based on your win/loss ratio when you hit 30? Is is based on how many games you've played? Or is it all just one big pot being churned around by level 30 with low restrictions on who you can play with? I'm not particularly upset by this, but it's just really strange as I'm not really that good.