Haven't Got an S grade since patch 6.6

I don't know if there is a bug with my client but i haven't been able to get an S grade since the 6.6 patch no matter how well i perform its always been A or A+. I noticed this when in my most recent game i got 15/4/15 on wukong and i was almost carrying my team I got an A+ for it while the mid lane azir who gets 13/10/22 gets an S+. That not the only game it also happened when i got a 8/1/4 on trist i got a B+ while the jungler Vi who got 9/1/4 gets and S. It really annoyed me, if the grading system is being used to see ur performance shouldn't it be fair? If it was a one of incident i would'nt have posted this but it has been happening game after game players who have similar performance and even under performing are getting an S grade while the only grade i get is A+. There is something wrong here and im really not happy about plz fix it riot.