Can someone try to help me understand this?

So let me preface my question with a quick summary of my situation.

During S3, I never left Silver 5. The last 2 months or so of S3 I had a massive losing streak that was so bad, I was getting 5 LP a win and losing 25 for losses. I somehow crawled my MMR back up to a respectable amount where I was getting 15 LP a win and only losing 20 a game. I went 5-5 in my S4 pre rank set and was placed in Bronze 2. I was pretty sure thats where I deserved to be after going only 5-5.

This passed season however was a completely different story. I started out at Bronze 2 like i mentioned and I fell all the way down to Bronze 4, before winning something like 17 of 20 games which shot me straight up to Silver 2 Promo's. I floated between Silver 3 and 5 for the rest of the season. This passed week I finished my Pre Rank set at 7-3 and was placed in Bronze 4.

This makes absolutely no sense to me. I have a friend who finished at Silver 3 and went 7-3 as well and was placed in Silver 5. Ive heard that each of your 10 games of your Pre Rank Set has an MMR attached to it that gradually decreases each game you play until you've reached your 10. But I dont see this written anywheres to confirm.

Let me emphasize, Im not salty, Im not butt hurt, I crawled my way out of the bottom of Bronze once, Im certainly better now than I was before and more than capable of doing it again. Im certainly better than Bronze 4

How exactly does the Ranked system calculate stuff. Cause trying to understand figure it out on my own just aint working boys.