can someone explain how this matchmaking process works?

so i get into a game trying to get an s, im all pumped up ready to carry if i have to. i look up my match on lolskill and see that my team is all bronze no big deal. but then i see that the other team not only has only one bronze but has a plat player in it. an all bronze team v 3 silvers and a plat player. im not complaining im just baffled on how in the hell this match up occured.