Maxtrix table Subtotal calculation when expand/collapse

So I have this matrix table like this. Let's look at Jul 22.

When it is collapsed, the Monthly On Time Completion % is Incorrect. It is supposed to be: 78/85 =91.7%. 

So why does it show 71.7%?  because when I expand it, it is the Avg of the individual rows:  90+25+100/3

So my question is, when I collapse it, how can I force it to show 91.7% , intead of 71.7% (as in the first picture)