Ranking System is Jacked

Today I played ten games with the same four guys. We lost two games total, one around game 5 and another game 9 against champion tier players. The lat game we played (10) was a clutch win against high onyx ranks. However… the ranking system gave my team all Onyx ranks except me. I got placed into Diamond 1. We all performed equally well and played as a team. Why would this Win/Loss Ranking system place me into a lower tier than my teammates seemingly for no reason at all?