TL:DR I suspect stomping = insane kda but often slightly less cs and the system takes all things into consideration still and little difference in consideration between a good and an amazing kda
Although S in short games are certainly attainable I find them easier to get if your simply doing well as opposed to just stomping.
If your just doing well, have a respectable KDA, helped with several objectives, been warding a lot and have been keeping up with / excelling in CS you will almost certainly get an S because your doing well in all regards.
I find in games where your just STOMPING you tend to be more likely to get an A than an S because while you push your advantages and often doing amazingly well in some categories but worse in others. (Often CS sacrificed for more kills) Not to say your not CSing but its something like instead of you getting 20 cs in a shortish time span and your opponent getting 14. Your getting 12, your opponent is getting 5 but you picked up 2 kills and a tower. Your getting less CS yourself because you are just denying + farming the opponent because you're just that strong. Your'e spending more tiem roaming and picking fights / objectives than sitting in a lane last hitting. Sure what your doing is right for closing out the game but one of the considered categories for an S rank falls behind relative to the average "good game" Bringing your grade down even if you did the right things. (just an imperfect system) It tends to balance more often than not if the game extends.
An example i'd use is one of my last Jhin games.
27 min.
18/4/14 136cs 71% kill participation. 10 wards placed (team range 8-13) 2 vision wards - A+ Grade
support. 6/2/23 29cs 64% kill participation (13 wards, 8 wards destroyed, 1 vision) S grade.
Looking at my stats in a nutshell i did pretty damn well if i do say so myself. The one area that was slightly lacking was my CS. Because I had several occurrences of: , walk out of base kill 2-3 people push 1-2 waves with team, tower -> drag recall. Slight variations but the early grouping team pushing killing etc.. meant my cs fell below where it normally should be but we closed out the game with the kill gold.
Obviously the one role that doesn't really suffer this is support because since they don;t cs much they don;t necessarily loose out on anything from early grouping,
