Getpivotdata pulls back specific elements of a pivot table basically using the name of those elements. It isn't designed to do separate summarizations of the pivot table - the pivot table is used to do that and getpivot pulls that information back.
from help on getpivotdata:
Description
Returns data stored in a PivotTable report. You can use GETPIVOTDATA to retrieve summary data from a PivotTable report, provided the summary data is visible in the report.
Note You can quickly enter a simple GETPIVOTDATA formula by typing
= (the equal sign) in the cell you want to return the value to and then clicking the cell in the PivotTable report that contains the data you want to return.
Syntax
GETPIVOTDATA(data_field, pivot_table, [field1, item1, field2, item2], ...)
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Regards,
Tom Ogilvy