Perfect game, but no S. What will it take to get an S ?

Well, the title speaks for itself. I expected an S (with a chest) at the end of the game, and was shocked to find that my two teammates had gotten an S and I did not. Even if the scores do not speak for themselves, my teammate Vlad was losing hard to Swain up until mid late game when we started to team fight more. I do not understand why I did not get an S. I expected an S+ and somehow got A+... All i have now are