I may be wrong, but from what i understand, cause preseason is funky, this is how it works.
When you FIRST do your placements on a account, you are on a clean slate.
But when you go to the next season, your ranked (MMR from last season/preseason) is kinda "curved". Like that is what you will be placed with when you do your placements again next season. If you go like 7/10 wins, you will be around the same spot, but you go 9/10 or better you may go up, but if your 1/10, you will be in bronze 5. Its really funky at the start of it, its best to wait a month after the season starts, to do your placements, so all the people are placed, and you get good "true" team mates at your elo.