Generally it will default to SUM if there are values in the field but, if I recall correctly, it will default to COUNT if the data in that column is in text, or if you already have that field in the rows section as well.
Regardless, the default is just that, a default. It's easy to change if Excel gets it wrong:
- Right click the field on the table
- Choose Value Field Settings
- In the "Summarize Values By" list, choose which you want. (Sum, Count, Average, etc...)