Pivot Table - false filter display

I have a pivot table that is pulling from data on a seperate workbook.

the data has a field that calculates the month based on a manually entred date for each record.

The pivot table brings this data in and has the month as one of the filters. When we crossed from may to june and i refreshed the pivot, the month filter has "6" listed twice. One "6" displays may's data and the other "6" displays June's data.

Rebuilding the pivot corrects the problem, but i am about to hand off this report to someone else who is just responisble for refreshing the data and has no idea how to build a pivot.

Any ideas?
I have already cleared the pivot cache to no avail.

no go....i have previously looked at those options on the table and manually cleared the pivot cache.