Unfortunately, the word count shown in Windows Explorer is wildly inaccurate. You can test this with a newly created file; the word count in Word is accurate, but the word count in Explorer is always wrong. I tried to find a pattern with Explorer's word count, but after I noticed that the word count changes based on the LENGTH of words you used, I'm just going to say there is some low-level computation that Explorer is doing that can never accurately count the words in a Word document.
Depending on the version of Word, the word count inside Word itself might not be accurate, as well. Older versions of Word did not count words inside textboxes, headers, and footers, but Word 2010 does include these words in the count, but offers a choice to not count these (if desired) by unchecking a checkbox in the Word Count dialog.
However, there is kind of a work around to get Explorer to display the correct word count. If you open a Word document and then go to the Review menu, and click Word Count, it will open the word count dialog and show you the accurate number of words in the document. If you then close the word count dialog and save your Word doc (Ctrl-S) and close the doc, Windows Explorer will now show the correct number of words in the word count column.
I wouldn't recommend this workaround, as you won't know which files you have opened and saved correctly, and as soon as you edit a file you will have to remember to run the word count again and save.
There is a software application called Total Assistant which claims to accurately count the words the MS Office ignores, giving you a word count for your files that you can trust. In my experience, it has given very accurate numbers as to the word count of my documents.
Check it out for yourself: Total Assistant Website.
It's very easy to use, you can even drag and drop files or folders to check the word count.