Showeing the overall grade distribution is a feature, not a bug. At least that was the thinking when this was implemented, but I can see why you might not want it.
I think the averages in the table will show a similar issue.
As others have said, this can only be changed with code modificaiotn in mod/quiz/report. (Should not be too hard if you konw a bit of PHP. Basically you just need to remove som chunks of code.)
Alternatively, if this does not need to be secure. You could probably do it by adding some display: none; CSS rules.