How on Earth does the Ranking Work?

Playing SWAT with my cousin. He’s ranked Onyx and I had yet to play it. Played my first 10 qualifying matches. Every game I went positive and every game we won against at least two Onyx tier players. So tell me why the heck I got ranked in Diamond Tier when my cousin got ranked in Onyx to start with and he lost some of his qualifiers?

I just want to know the general mechanics behind this as it would explain why the Onyx tiers we were playing were not all that stellar aside from a few as well as why some platinum and diamond tier players consistently perform poorly as well.

Does anyone know the general way it works?