Why I can't get an S

I played as Aatrox {{champion:266}} in the jungle. And my stats were 13/1/6 when the game ended (20:38). And the team score was 27 vs 3. And I dealt most dmg to champions and I also had highest total dmg. And still I got an A+? These are my best stats on this champion... * What do I need to do, to get an S? (I'm only missing one S to get Aatrox to lvl 7) My build: {{item:1416}} {{item:2031}} {{item:3047}} {{item:3153}} {{item:1011}} {{item:1029}} https://gyazo.com/e4031115ebfaa26c2b10e50ea04aa96d
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