Why is the word count on Pages inaccurate?

The word count of Pages does not seem to accurately count words. I noticed this doing an assignment for college. With 595 words written I added the sentence:

"Massage is an ancient form of therapy, with origins that may be traced back to ancient China and earlier."

When I counted the words in this sentence (19) and added them to my previous word count, I got a figure of 614. However the word count Pages gives me is 613? Why do they not add up the same? How can I do an accurate word count if Pages gives it wrong?

On a similar note, doing an assignment before Christmas, I was trying to shorten my word count from by 600 words. By just opening the document on a Microsoft Word program 500 words vanished. Does anybody know why this is?