For those of you playing with PowerPivot or regular Pivot Tables in Excel, CFO.com has a look at Why Pivot Tables Won’t Sum. Essentially the article tackles the question of why some numeric Pivot Table fields default to count instead of sum.
Spreadsheets: Why Pivot Tables Won’t Sum
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.