How you find out that the Grading system in league is a Joke....

Your farm is sub-par, and you only placed 5 wards. The warding is the bigger part.

Your CS is really low for that long a game. Warding plays a part, but CS matters a lot more in determining your score.

My advice is to farm more and put down vision in the enemy JG. But S scores are honestly meaningless outside of getting your mastery up. Getting an S+ doesn't necessarily mean you did the best thing possible for your team (Like taking all your team's farm so your CS is caught up for an S+).

Yeah the grading system is very poor especially for instances where doing well doesnt coincide with typical gameplay. Like i nyour case you were likely busy in lanes getting a kill every 2 minutes instead of having time to ward and farm which ended up working super well for your team but the grading system is too ass backwards to see that. Also try playing a pure enchanter who does like no damage and gets like no kills and youll know how ignorant the system can really be.