If I understand your question:

You are saving a copy of an Excel spreadsheet (or Word document) to the database.

You then open that Excel spreadsheet, make no changes to it, and Save it.

You are now surprised that the checksum of the file is different than it used to be.

That's most likely because the files are different.

Just now I made a copy of an Excel file. (Just a plain file copy, nothing to do with SQL Server).

I opened that file in Excel.

I did absolutely nothing other then press Save, and then Exit.

I compare the file against the copy I just made, and I see that the file has changed.

It's not surprising that the checksums on the two files would be different, since the files are different, even though the two files hold the same "content".