Mastery system

Sorry for my bad english, but i have to ask why in the game i just played i didnt get a least a S, I mean i played zac jungle tank and did a good match helping my team mate to get some kill and focus objectif and not being an d*ck. At the end of the game i finish with a -S with the score of 19/1/24 and my friend who was playing lucio adc got mad and die a lot early game, but with my help got back in game with the score of 11/8/9 and finish with the same -S as me. IT is because playing tank you need a better score or something else i dont know. Please someone tell me why we finish like this because it really hard to get the mastery 7 with zac tank like this.