When I add a base measure with average aggregation, the Total field should show the average value. But why is it showing the sum of that average?
The sum of the average is displayed in the 'Total' field becauase the functionality is to display the total value and not the average value. When you add a base measure with average aggregation, it considers only the line items and not the 'Total' column. The total field always displays the summed - up value of the line items.