Excel 2007 Pivot table cells number format changes w/out user inpu
 

I have a multi-column pivot table that includes columns on summed file sizes,
file counts, and the sum of a currency column in the data rows. The file size
columns have a custom format that looks like '#,### ("MB")'. I switched the
currency column to use $ formatting by selecting the column and changing the
format, but every time I change anything about the table (sort order,
collapse state on a roll-up, for example), the currency column reverts to the
custom format. Any ideas as to why?

The workbook is in xlsx format.

TIA,