Normal Matchmaking

How exactly are games matched for normal games? I'm unranked and have been for 3 (or 4?) seasons now and it seems recently that the draft pick has left the game feeling one-sided from the start. I usually queue alone or with 1 to 2 friends, never a 5-man. Recently my games seem to be myself and 4 random players with assorted ranks against 4 or 5 mans that are all highly ranked (ie plat+). I just played a diana game against a 5 man that had a Master ranked player. I'm just confused why I'm being matched with these players - they are much better than me . Is there some secret bias against me or is it just random? I doubt it is but help me understand if you can. Thanks :D [slayer-jinx-wink]