Ranking system

http://i.imgur.com/E7YK4VK.png I mean, wtf? I participated in 31 out of 33 kills, didn't die a lot. I was sure I'll get at least A+. No, %%%% me. Should it be 28\3 to make it to A+? Why ranking system is so confusing? Sometimes it's 1\12 a+ and sometimes 28\11 b+. And since riot love to hide everything from players (about bans, ranking etc) - how do I know I'm getting close to S?
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