S+, is this allusive grade a myth?

Rito, I am very displeased to have had this post game let down. I was told Nunu is not meta right now, I was told I could not suffice as a proper jung, and I set out to prove them wrong. I spent the game warding like a madman, taking objectives like an American couponer, and providing CC like a real Nunu should. I did not die once Rito, not once, and I destroyed the enemy team along my brethren in combat. YET, as soon as I clicked the continue button on our beautiful triumph, I was shocked to have received a mere A+, when the support, just about equalling me in usefulness received an S+. Please explain to me this grading system, for this time of debauchery will not be accepted on my desktop.