Pivot Table issue --variance columns

I've got this pivot table, and for column headers I've got different weeks, like 821/11-8/27/11. Each week the pivot table gets updated and a new column gets added, and I'm wondering if there's an easy way to get a variance column between each of them showing the variance between any two weeks. Since I'm not trying to find the variance between two different fields, I haven't had any luck using the Pivot Table formulas---each week is not a separate field in the data set that is used for the pivot. There's just a field called 'week', and the values of that field form the column headers in the table. Is there any way to do this easily inside the pivot table?