I have created a pivot table that refers to a database in another sheet in
the same workbook (Excel 2007).

Often, when I refresh the pivot table, it returns a certain result. I then
refresh it again, almost immediately, without any changes taking place in the
source database, yet the returned data in the pivot table changes!

Why is this happening? I can't rely on the accuracy of the data if it
changes every time I refresh the table.