Champion Mastery System

Okay so I am the Riven and my friend is the Katarina. He was 17-0 and I was 7-4. At the end of the game he got an A+ and I got an A- . I remember because in chat we were like ''LMAO RIOT PLS'') He still does not have a single token on Katarina because of this. Regardless of how well he performs he just does not get anything above an S- and that game was not even an S- according to the system... I don't know how that is happening but it is definitely something that needs to be looked into...