Why Report Custom Summary Formula 'Subtotal' or 'Grand Total' Has Unexpected Results

Typically, when working with the subtotal and Grand Total values within a report, the calculation returns the sum of the values for the prior groupings. With Custom Summary Formulas (CSFs) this behavior is handled differently. For Custom Summary Formulas, the evaluation is instead based on the sum of the contributing values. These summed values are then used by the formula.

For example: 
Formula is Column A * Column B 
 

The traditional expectation in this scenario would be that the Subtotal would result as a sum of the prior formula results. This would produce a calculation of (6 + 12 + 8=) 26.  This is not the case when working with Custom Summary Formulas however.

The result is based on the sum of Column A multiplied by the sum of Column B. The calculation being performed as ((2+4+4)*(3+3+2)=) 80.

This is the current product's expected behavior.  To vote and comment to have this behavior changed, please visit the Salesforce Idea Exchange, Fix Reports Grand Total Calculation Error.