so what is this flex matchmaking based off?

im s3 i got into my promos for s2 and this second game i was matched with high golds some plats and a diamond player..... 3rd game i was matched with a team of 3 plats and a gold vs an entire team of plat players. IM SILVER. i do like the challenge but the fact that in my promos im not against well high silver low gold players in kinda insane.