My MMR shoots way down?

I'm playing ranked for the first time and I'm on my 9th game, last one before I'm ranked. My MMR places be around high silver, like silver 1 or low gold. My last game is a really good win, given an A as support. Because of this win, I'm expected to be placed in high silver or low gold. I'm placed in silver 3 and my MMR shoots way down to meet the silver 3 average. Not saying getting placed silver 3 isn't bad for my first time, but I'm just curious why I didn't get placed higher.