Got an S- with Kassadin, but it shows season highest as a C+

I went 17/4/5, destroyed 5 turrets, and 2 nexus' and with the last 9 kills were on a kill streak I got an S- when the game ended The rest of my champs show their highest grades: Galio S-, Ziggs S-, Master Yi S-, Zilean A, Wukong A, Teemo C+, but Kassadin only shows a C+ that I got from a couple of days ago instead of the S- minus I got on Dec. 6, 2016