I'm not claiming to be an amazing player, but with most characters it seems far easier to get an A, much less S. I have received other scores similar to this one and also never got A with this character.
Your cs score is incredibly low for the time that game took, and your gold income wasn't high compared to several other players.
This game is more than just KDA, which these grades kind of point out...
Edit: 7 wards in a 48 minute game? Where is your excuse for that abysmal warding?
Well, it compares you against other Chogaths, and it seems like Cho gets pretty good KDAs most games these days.
For another example: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1815639615/227556851?tab=overview
Again low CS and wards. Regardless, the twitch gets an A, apparently based off CS, regardless of how much more expected/easier that is for an ADC.
