Cópia do link (vale a pena ler todo o artigo da base de conhecimento):
This behavior occurs when something in the file has changed. Many times the user doesn't realize there are elements in the file that have been updated or calculated. Here are some examples of common scenarios:
- There is a volatile function used in the file. Examples of volatile formulas are listed in the more information section of this article.
- The file has been opened in a newer version and formulas have been calculated. See article in the more information section.
- A formula that contains a link to a formula in another workbook and that uses volatile functions.
- Linked pictures.
- Iterative formulas.
- Charts that are on chart sheets or that are embedded into worksheets but that have their source data in another workbook.
- VBA code that updates the workbook."
Esta lista contém os casos mencionados em outras respostas como possibilidades, e alguns outros também.