I had the same problem, and after several hours reading through several forums and pages I found the root cause of the problem and solution, so I'll be copy&pasting my answer on the forums I read.

There seems to be an issue with excel files, in particular Excel 2003 xls files, whenever they get opened in Excel, even if they don't get changed and don't get saved, Excel automatically updates some of the file's metadata, such as the "Document Properties and Personal Information" and "Last Accessed Statistics", therefore the file every time it gets opened changes a little bit, and this makes that the MD5 changes also.


To learn to view the advanced properties for an excel file:
http://www.ablebits.com/office-addin...nt-properties/


Solution:


One way to avoid this problem is to remove "document properties and personal information".


Remove hidden data and personal information from Office documents. Excel 2007:
http://office.microsoft.com/en-us/ex...010037593.aspx


Remove hidden data and personal information from Office documents. Excel 2013, Excel 2010:
http://office.microsoft.com/en-us/st...010354331.aspx

Other way to avoid this would be to use xlsx files. I have been trying to replicate this behavior in xlsx files, but it seems it only happens on xls (2003).

Regards,

Caridad J. Valenzuela R.