Are you saving the doc in word?
Then word is probably updating the author information, and storing a different date in the doc file.
Word is a bloated format, full of time-dependant information.
Also, old versions of MS-word are known to copy random parts of your computers memory into the doc file to make the file larger (don't ask me why, it was a security issue some time ago).
That would make your new file different from the old one.