What did I do wrong?

Please someone explain me how this performance system works. It is almost impossible for me to get S or higher rating when I play support. Just now I had a game where my KDA was 27, placed 3 times as many wards as my teammates, was present at most tower and inhibitor kills and still only get A+ rank. While my teamates get S and S+.... Can someone tell me what did I do wrong? Also my previous game I finished 4/1/14 as Sona, earned more gold than most of my enemies, placed twice as many wards as others, and still only get A+ rank. Am i playing support class wrong? is it because I died once with a paper support? Is there any mysterious thing I should do to get my weekly chest? Not to mention I filled in both of these draft games. This system is seriously stupid and unfair towards supports and those who fill in games...
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