You can use underlying or surface values when working with totals in Pivot sections. Underlying values refer to values from the original results section. Surface values refers to values in the actual report section. The two approaches yield different results, and produce values that may be displayed incongruous with the values in the report.
To understand this difference between underlying and surface values, consider a simple pivot table with two values of 20 and 30. Each of these is already a total of underlying values (20 = 8 +12 and 30 = 10 + 20). An average of the underlying value yields the result of 12.5 = (8 + 12 + 10 + 20) /4). An average of the surface values yields the results 25 = (20 +30) / 2). By default, the surface value feature is not active.
To activate surface values, select a pivot item and