My friend just got placed into Bronze 2 as well. I was duo with him for all of his placement games. We won 9/10 as well. last Season he made it up to Silver . Btw, he plays support...
Honestly, "placements" seems to have no relevance to where you get placed. We are still under the impression that doing well, or poorly in your placements, has some sort of influence in where you get placed. Many people like you and my friend try their best in placements in hopes of being placed closer to where they ended up last season. Under the impression that doing well, means you will get placed higher, and doing poorly, will place you lower.
I think it's more complicated. A better indicator of where you get placed is probably your MMR at the time. It seems to have little to do with your WIN/LOSS ratio during your placement series.