Riot Please Answer Champion Mastery on Grade

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2206683997/264001?tab=overview

Can anyone explain to me how to get an S+ mastery? How do you get an A+ with twitch's score when he participated in all the team fights and everything else? The reason why i ask these questions is because it says that it will rate your performance in team oriented aspect and i don't think it does. i think its busted almost all the time. Can anyone further explain this to me? Please and Thank you =)