I just played a game where I got quite fed really early as Ahri mid, but the enemy top and bot lanes were doing really well so it was a close game.
I was CS'ing fine/great for the first ~15-20min of the game, but I knew I'd hit full build early that game. I did indeed finish the game (with a win) at full build + sorc elixers and gold to spare.
I ended the game with a great score and was expecting an S+. Now, before all the haters start downvoting me, I ALREADY HAVE MASTERY 7 ON AHRI - I really don't care that I only got an S-.
That's what happened though. I carried the hell out of the game. Had by far the most damage (58k for me, the enemy ADC had 46k, my ADC and the enemy top each had 34k). Had about 90% kill participation, and about 50% of the total kills.
And since I knew I was going to be full build, I told top and adc to take all the farm. I purposely hit the wave low and didn't last hit it so they could get farm (except when we needed to push).
My top laner finished with 100 more CS than me. He got an S. I got an S-. Despite me being 22/5/19 (41 kill participation of 46 total kills) and him being 5/3/15 (under 50% kill participation). Despite me having more than 2k more total gold than him. Despite the CS disparity being intentional because I didn't need any more gold.
I get WHY it happened - my CS was garbage at the end of the game. For about the last 20min I didn't do a last hit aside from getting blue buff. Seriously though, it was THE RIGHT MOVE that game. My top and ADC each finished the game with 17K gold. That'd have been about 14-15K if I had kept last hitting since I could insta-clear waves with Q-W (I'm also implicitly including gold they got from having the earlier gold and being able to get kills in fights I would have had to get the kills in otherwise). That's a full item on each of my two main ancillary carries, because I purposely gave them CS I didn't need.
I had the entire focus of the enemy team all game. I took every ultimate they could throw at me, every summoner, and still survived the huge majority of fights (and got 2-3 kills out of the deal each time). I caught the enemy champions repeatedly with vision from my wards and ult'ing over walls to charm them and 100-0 them. I focused objectives all game long. I went on a 13-kill streak. I had the presence of mind to give up most kills late game without allies dying because I knew I didn't need the gold but I needed fed allies. The game was the definition of a carry.
I think games like this prove the mastery system really has no clue what it's doing. It can't detect when missing CS is a good move. How could it? Why wouldn't you want more gold? Oh, because your ally needs it more? Irrelevant...?
As I said earlier, I don't care that I didn't get an S+. I already have Mastery 7 on Ahri, it makes no difference. I was just shocked. I mean, I looked afterwards and it was pretty obvious why, but at first I was really shocked. Like I said when this system first came out, it's going to encourage suboptimal play. People told me I was full of it - that if you play well you'll get an S+. No, stealing kills and getting as much gold on yourself as possible is how you get an S+ but not always how you win games.