autofill

so what is the actual coding or percent chance that one gets autofilled? Personally I find it completely unfair. I have played roughly 80-85 games this season and 8 of them I have been autofilled. Thats 10%. I've won 0 or 1 of them because I don't play support. Riot asks to select 2 roles and I do. In fact I can play 3 roles, but i don't play support. So why am I having to play a role I know I'm going to lose in ranked (the place where you are suppost to play your best champs/roles). 10% is a huge amount in regards to win/lose % (thats about 20% winrate-wise 8 less loses, 8 more wins 16 difference 16/80 = 20%). Aside from "better que times" which to be honest isn't really a factor until d4+, I see no reason.