Poor mastery grade even though you did well in game?

Just played a game as Lee jungle, and my end kda was 2/3/21. I layed down 32 wards, multiple pink wards and cleared 6 enemy wards during the game. I gave dragon gold to my mid and adc when possible, and threw down a few plays that won us teamfights. I ended up getting a C+. Haha I'm curious why? Is it because I only had 2 kills? or because I bought a sightstone as a jungler?

I know it doesn't matter at all really, just curious. Has anyone else received a mastery grade that was far lower than their ingame performance?