Excel pivot table calculated field total returns unexpected value

Question

Can someone please tell me what I'm doing wrong? I have created a pivot table based on the following data for each month over a six-month period: Month, Impressions, Clics, Joins, Acceptation Rate.

I would like to display in a separate column the number of Valid Joins, which is JoinsxAcceptation Rate. So I created a new calculated field with the following formula: Joins*Acceptation Rate.

This calculated field works well for showing the valid joins for each month but my problem is that, at the bottom of the Pivot Table, the "Total Sum of Valid joins" row that the Pivot Table generates automatically is not properly calculating the sum of the values in the calculated field. It says 15253 instead of 1387 (=Total Joins * Average Acceptation Rate = 1508*92%)

Any suggestions? Any help would be greatly appreciated! Sylvain

Answer 1

1    |      100 | 90,0% |  90
2    |      900 | 10,0% |  90
    ---------------------------
total|     1000 | 50,0% | 500 calculated using average rate
total|     1000 | 18,0% | 180 calculated DERIVATING average rate

It is obvious that Total Valid Joins <> Total Joins * <simple> Average Rate! Indeed, Valid Joins = Total Joins * <weighted> Average Rate!