How did I get placed so low?? A giant slap in the face...

I finished the Season in Plat 3. When i played my promos at Plat5 it jumped me up to Plat 3 ( I guess because my MMR was of someone at Plat 3 levels. I probly could have gotten higher as i still had a 70% win rate. I stopped at Plat 3 and never played till the reset. I played my placements over the last 2 nights and did very well with 7 Wins and 3 Losses. 2 of which were my last 2 games. So can someone explain to me how Riot felt i should be placed in GOLD 3...... I am embarrassed to say the least. My buddy finished Plat 3 as well and after he won 6/10 he got Gold 1. If I look on OP.gg right now it says my MMR is around 1900 which is WAY HIGHER THEN GOLD 3. This makes me not even want to play this season and I don't feel that this is fair... Has anyone else had these placement issues??