I assume this is a bug in Excel 2010:
Every once in a while wierd things are happening in the first column of my PivotTables when the first column is a Row Label.

* Sometimes the number format is changed for some of the values. Say that my source data column consists of numbers with no decimals, then if I in the PivotTable format the column as number with two decimals, only some of the row labels get two decimals.
* If I happen to enter something in one Row Label cell, it is not possible to refresh the table to get the real value back. Nothing happens.

I have found one and only one way to solve these mysteries: Select one cell in the first column, right click->Group and then choose whatever kind of grouping. Then ungroup again and Abracadabra - things are OK again.
Any more information on this phenomenon?? Anyone else who has experienced something similar or even identical?