Open que taking my lowest role que?

I won 7 of my 8 open queue games with a friend I mostly played healer (he played DPS). A multitude of the games we played were complete wipes I mean they got nowhere close to winning and I got average healing done and by no means was pocketing him. He did extremely well and when everything was said and done our last 5 games together were all wins. We then were placed in bronze and silver which are both of our lowest ranks in the role queue. My lowest is tank, healer and DPS are gold, his DPS is plat and he is silver in support not placed in tank. I have easily performed worse in my role queue matches and placed higher so why am I placing so low?