How Does This Grading System Work???

So I've had 2 lvl 7 tokens for Kat for the longest time and I've been spamming Kat sooo hard for the past few days trying to get an S. I've been getting scores around the lines of 12/3/4 and getting so many S-. I was okay with it at first but it was still annoying. The game I just played with Kat I went 20/3/3 with 122 cs and I also warded all game. Once I won that game i was super excited thinking I was defiantly going to get an S for that game. Buuuut when the end game screen came... "Katarina has scored an S-" After that I defiantly got super salty. Could somebody please explain how this stupid grading system works? The only thing I could think of was that I didn't have that many assists, but 20 kills should make up for that...

                                                   Even a link to some video or other post explaining how the system works would be appreciated. 
                                                                                                                                                                                                                            ~TheCharmingCharm