I get the same behavior with your document in Word 2007. If I go to print preview and back, the number is updated. If I open the footer for editing, the number is updated. If I print the document, the numpages field is updated.

I created a new document with new content but a page x of y footer. I closed it. Upon reopening, the numpages was wrong, however, after I scrolled through the document and came back to page 1, the numpages was correct.

This may have something to do with the speed at which Word is loading your document from your network.

If it prints properly, why are you concerned?

Page Numbers in Word 2007-2013