Watermarks are anchored to headers in Word. From your description, your
document has at least two different headers (possibly because of a section
break on the next last page). Place the insertion point on a page where
there is no watermark, and insert the desired watermark.

For more on headers, footers, and sections, see
http://sbarnhill.mvps.org/WordFAQs/HeaderFooter.htm and
http://word.mvps.org/faqs/formatting/WorkWithSections.htm.