The Grading system is kinda crazy lol(might be bugged for me)

So when the champion mastery system was introduced I was thrilled to have the chance to show of my love for my favorite champions in the form of sick mastery levels. Since the release of hextech crafting I've gotten three champions to lvl 6. Sion,Jhin and Tahm. Two months ago I finally managed to get two s ranks for lvl 7 on sion and then something bizarre happened. No matter how well I did I would never get an S- let alone an S. It seems that for me the game has glitched and locked my score on sion to A+. I checked the games where I got S ranks on Lolskill and the average is to perform 100% above a regular player with that champion. Taking into account Average cs and kda. So I experimented a bit. The games I got S ranks on I did 120% better than the average player. Over the past month I've had six games where I've performed 150% above average. And all I've gotten is an A+. Sometimes even when I end up doing badly on sion I get an A+.(Like very frequently about like 90% of games overall I end up with an A+ I didnt deserve,I legit went 0/5/2 as sion support and got an A+)Same for jhin(went 18/0/9 with 160 cs and got an A+,this month also) Anyone got any ideas what's going on here?