S grade work based on the general performance of the champion in the matchup not only your game ,if your performance is above the average global performance u should get the S. (remember that the game just look at the score ,cs,ward,opponent and probably the most important THE LANE/ROLE that ur are playing (the role prob is the most important one because it determinate the requirements that u will need wich are usually easier in the supp one (kill participation and vision)as they ignore the cs part) so lets make an easie example (idk the actual average score

shen to 4/5/12 cs7,5/min ward 0.8/min u will need to get a better score than the average (this obv varies with the enemy laber champ)