Excel 2007 :: Add Suffix (1) To Filename When Saving?

I find, when performing a 'save as', xl puts in a default filename with a suffix (1), or (2), or (3) and so on, to the existing filename, after I have opened a file from MS Outlook. I had assumed it was because there was already an existing file in the default directory, and so it was creating another version, but no other files with the same name exist in the default directory being saved to, or on the remainder of my computer, so I don't understand why this occurs.