Percentages are Sums instead of Averages on the Total and Averages Rows

It is a field property.  But simply clicking the field to show averages instead of totals may not result in an average which is mathematically correct.

You will get an average of the averages but that does not mean it's a proper weighted average of the total. It could literally an average of each line item but one line items could be for 1 million widgets and the other item the other line item could be for 1 widget and they will be treated equally.  

There are more advanced features of summary reports which may allow a proper calculation