I genuinely believe you have enough data to support what stipulates a great match versus an average one that doesn't have to rely on historical games of a player doing well especially when that data could potentially be skewed when a champion is strong in the meta during a certain patch. I do not agree that I got less than S- in this game when I was a significant reason that the Ashe got one.
i was skeptical at first, but imma have to hard upvote this. i main nami as well:
https://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/3205022513/233787954?tab=overview
this is from my match history, granted it was a little while ago. (before support item changes, about 2 weeks ago)
OP's game lasted 35:47, and he got a KDA Ratio of 6.6 with a score of 3/5/30, placed 26 wards. (not sure what he got, but im assuming less than S-)
Mine lasted 34:04 (so the OP is literally just a minuet longer) with a score of 3/5/27, which is a KDA ratio of 6, i placed 37 wards.
i got an s- this game.
edit: proof it was s-
https://cdn.discordapp.com/attachments/505479052396855309/648720449907327016/unknown.png
You are compared to the rest of support namis in that queue for your MMR. If you are not in the top percentage for S- you just don't get it.
And of course the champion's strength is relevant because the stronger the champ the better the average and top 10%(for example) namis are.
