Why Rito

How Autofill works. I played ranked and got a supp role and I was playing because we had 3 assassins and I was the only tank. then I got the ADC that I had and I played the game. then another autofill and again supp. I lost again then I got the JG I had as a secondary. and I lost. will someone tell me how it works ??? because I was set up because my bot has 0/4 in 5 min. And I'm supposed to play supp. The man who got me when I got s5. And S5 I have because I did 4x promo when not more and still feed me top. GG RITO