Ah yes terrible placements

I don't know if anyone else had this prob but I ended last season silver 2 (not the greatest but still) and I had decent MMR making around 23 per win so standard gains and yet when I went into placements and played my first game and got an S- and had the 2nd most dmg in the game as a jg while also having decent cs and getting jg objectives I get placed in iron 1. Someone please explain, this happened last year as well and I easily climbed out of bronze to silver so I know my skill level is at least high silver. Someone please explain I don't get this sh#@.