I am sure another Placement game question.

So here I am bronze 5 ... again, what ever i moved out of this cesspool into the deeper s5 cesspool multiple times. Here is my question. I finished s5... enter my placement games go 6-0... ooh ooh ooh i am getting excited!!! Ended up 7-3, not the best but pretty good at my mmr with the toxic flaming afk and " first time - enter champ name-" that seems to just fester here ( i know all ranks have baddies but the section of bad pie in this mmr is much higher). So the final game ends and i am looking forwards t s5 again... maybe s4 or even s3, i know from two years not to get my hopes up to much. So low and behold my surprise when bam Bronze 5 hits my screen like they are giving me a present. Can someone please explain to me the actual math. Also i request a bit of transparency in this part of the game from riot.