This sounds very much like a problem I have seen documented for earlier
versions of Word but supposedly cured.

I created a new letter template with 3 sections.  1st and 3rd sections were
locked forms (for filling of names, addresses ref's etc.) and middle section
left unlocked for body of letter.  I used a NUMPAGES field in the footer.
The first time I used the template it worked fine.  The second time, a
simple 1-page letter, the editing screen showed field result as "1" and print
preview also showed "1" but when I printed it, it came out as "2".  After
about 4 different sequences of save/re-start lock/unlock etc. (and re-prints
each time), the field eventually came out as "1".

I'm running Word 2003 (fully updated) under XP Pro SP2.