Service Request resolution time

Hi,

I am trying to create a report with the cases resolution time - however ew have used 2 different date fields (so the metrics for average closing is not helpful).

When I manually try to create the report, I do the timestampdifference to find the number of days between the open and close date, and then either average distinct or Sum dinstinct however when I manually calculate the average (add up the timestampdiff values and divide by # of cases (filtered so that we are only looking at closed cases)) I get a different number to the avg distinct or even the sum distinct on the timestampdiff field.