Pages does not edit Word documents. Whenever you open a Word document in Pages, you launch a conversion filter that copies the document into a new Pages document. This new document is the document that you edit. The old Word document remains unchanged. If you save your edited document, then you save a Pages document. If you want the new document in Word format, then you must affirmatively invoke a conversion filter to convert the Pages document to a Word document.
It cannot be stated too many times that Word conversions are problematic. The most frustrating conversions are those between two different versions of Word on Windows. In my personal experience, the most reliable Word conversion was to open any current or older version of Word documents in Word 2004. Word 2011 has been widely praised for its ability to open Word 2010 and other Word formats. In my experience, Word 2011 is not nearly as accurate in its day as Word 2004 was in its day.