Can Someone Explain This.........

I'm confused on this ranking system since I started playing.... I know some one that was boosted to plat 5 they were found out banned for two weeks and reset did the placements went 6-4 placed gold 1. Seen someone go 10-0 in placements on a new account and was placed gold 1. I went on a 10 game win streak in solo/duo being almost 15 games won ahead of losses and still have lower mmr then where it is I am ranked. I don't understand this game at all the more I read about people's experiences.