Hi
When you open the sheet, if you have any of the volatile functions on
the sheet like TODAY(), NOW(), INDIRECT(), OFFSET() etc. and your
default calculation mode is set to Automatic, Excel has to recalculate
these. The values, other than NOW(), may well not change, but the
internal flag in Excel will have been set to say that a recalculation
has taken place since the last time the file was saved, hence it comes
up with the message.
If you know you have not made any changes, then you can answer No to the
prompt.