Can someone explain to me how placements work in ranked 5s?

So I just finished placements for a ranked team and was placed in Bronze 1. The only reason i find this to be strange is that we won 4/5 of our placement matches, and every game we played we were matched with were high mid-high platinum players and even a few diamond players. 3/5 games were stomps with us winning with 15k+ gold more than the enemy team and 10-20 kills more than them. I would love it if someone would explain to me why we got ranked the way we did. Edit: All 5 of our members are in gold 5