Got a questiona about placements

Hi, kinda mad here regarding placement matches. Alright at the start of season 4 I was placed into bronze 5, however I ended season 3 at bronze 4 so that's fair enough. I won 6 of 10 games that time. So I worked hard and got up to silver 4 by the end of season 4. Pretty happy with this, I went into my placements for season 5 feeling good. Won 6 of 10 again and got placed into BRONZE 5! Like wtf? I looked online and my MMR was above average for someone in silver, let alone bronze. So I'm here asking what happened? How did I end up in bronze 3?