S Rank?

I was playing Veigar mid recently (trying to get a level 6 mastery on him), and got a kda with 25/5/11, and team had a decent amount of cs, and got an A+ (Here is a link to the game: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2237644658/231129034?tab=overview) Then i let my friend have a go on my account with Veigar (when he was at my house, not account sharing), and got 20/5/10, and got an S-, with 40 more cs than the game i played. He also participated in more tower takedowns, and drag and baron. (Link to the game:http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2240557564/231129034?tab=overview) So I am assuming your rank is not only determined by your k/d/a ratio but your overall performance and team performance. In case I a wrong though, what would be the real reason why I didn't get an S, but my friend did? (I was also level 24 when playing, and level 26 when my friend was playing, would that have any affect?)