How do you get an S rank?

Alright, now this has been bothering me for a long time. How the hell can I get an S? It's always only A, A and A. You can see my last game here : Lux and Kha'Zix on my team earned an S. As you can see I have more placed wards, better KDA and almost as much farm as him - he has 25 more - and I get A, not even A+.What the hell ain't I doing right. I know that I get a rank compared to the stats of the people with the same champ in my region, but I don't think that they are THIS good. I rarely see any malzahars at all. I hope you can help me...
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