(Another) Question to the Champion Mastery

Can anyone answer why I get B+ as 0/8/29 as well as 1/0/8 Support? Both times my ADC carried the game. (1st one the enemies were bad, but the second one seemed very bad first 20min but my Vayne carried it.) I mean.. I got an S with like 7/5/6 or smth and a B+ with like 3/8/2 ... Slowly but steady I'm getting a bit p*ssed ...