Grading Is Not Right

So, I've been trying to get mastery 7 Nami, and i just had an incredible game where my score was 1/1/29 and i got an A+. i have had worse scores with supports like Morgana and i got a mastery 7 shard with her, like honestly?! I've been warding non stop and helping the team, where did i go wrong?!

https://snag.gy/jNMFe4.jpg <---- here's the proof, like did I need more farm or what ._.