There are several reasons why this can happen. To complicate matters, different versions of Word all seem to present their own unique problems. Rather than go through them all, here is solution that I found works the best.
In Word, go to View and click Header and Footer. This opens the Header work-area.
Click inside the Header.
Select and delete any table.
Tip: Tables are the source of most Page X of Y problems.
Close the document and re-open. The page numbering should be fine.
Even though you may have to redesign the header, at least you can compile the table of contents and continue working on your document.