Anyone have a clue how to get a Jungle Zac Grade over B+?

Note: This is not about S grade (If i need that I can always go Top Zac). Rather after numerous matches based on what I have seen I am thoroughly convinced that the mastery calculations for Zac are simply bugged.

Look at my last Zac game before posting. That was a B+ certainly the highest I've ever gotten for Jungle zac yet that was basically an absolutely perfect fantasy game (all 5 Dragons, Baron, Towers, Wards, Kills, Assits etc etc etc) and in general even when I have domineeringly good matches with Zac I commonly see B's and even C's. I think I once got a C- for a game that I solo carried.

I never have these issues in other lanes or with other champions. Even other champions in the jungle I easily get better grades, and heck when top lane getting lower then an A- seems difficult.