How does this ranking system work in games?

I just played a game where I went 24/7/14 as Lucian and I didn't get an S- and my teammate went Akali top and went 20/9/13 as Akali and she got an S- . I don't know how the ranking system works but I got more farm than her, more kills, less deaths, more assists, more turrets and inhibs destroyed, and more damage. Can someone please tell me how I can get an S- because I just want to get mastery 6 on Lucian..