Here is the answer to this question from Debra Dalgleish's excellent
Contextures site (http://www.contextures.com/xlfaqPivot.html#DefaultSUM):

Can I change the default summary function for data from COUNT to SUM?

You can't change the default settings for the data fields. If a field
contains blank cells, or cells with text, it will default to COUNT.
Otherwise, it will SUM. There's a feature that changes all the data fields to
SUM, in my pivot table add-in, that you can download here:
http://www.contextures.com/xlPivotAddIn.html

Hope this helps,

Hutch