i just don't understand the grading system

http://imgur.com/rUMIWsN i was 12/3/14 as kennen. No S. fizz was 10/6/16, S-. the taric being 7/4/25 and getting an S i can understand. i can also understand the 9/7/20 sion getting as S. but the fizz that gave my lane first blood, and got back to a positive kda from coming in last second to one hit my kills, i just can't. my aurelion sol was 3/1 when i was 0/0/0. despite all my mia pings and danger pings, everyone fed him and i was still able to come back to 12/3/14.