Word 2007 -- repagination not coming out the same way twice

I now have a 152,000 word, 21 chapter book as one single document, and
one thing I have been noticing some things that it doesn’t seem to be
coming out the same way twice after the times I see “Word is
repaginating” at the bottom. This happens every time I print out
short segment from the whole document, and every time I switch views
on the special character (and many index entries, and every time I
update the index.

Lines of text that appear at the bottom of pages sometimes appear at
the top of the next, and vice versa, and sometimes this results in
extra pages at ends of chapters where I have a next-page section
break, throwing the page numbers off for the rest of the document.

Is this something I’m doing and not knowing it, or is this deep in the
Microsoft arcana? I’m trying to put illustrations in the black spaces
at the ends of chapters, and can’t if the space keeps changing on me.

-Butterfly Bill