How did I only get A+??

I really don't know what went wrong. Kha'Zix (Jungle obviously) Score 27/3/11 Damage 54,377 (Highest in team) Objective damage 34,973 (Highest in game) gold Earned (23,122 Vision score 31 Is 3 deaths too much? I wasn't expecting S+ or anything but I rarely ever get such a good game. I really want to know what else I should have done for an S- or an S. (I really want chests)
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