prior version message

I and my co-workers often get a message when closing an Excel spreadsheet
that the spreadsheet was last saved in a previous version of Excel, would you
like to save it in the current version. This happens seemingly randomly, it
doesn't always happen on the same spreadsheet, but ofter happens on a
spreadsheet that I just closed myself - even on my hard drive, where no one
else has access. Does anyone know what is going on here?
--
-Hovis