Probably because you have the Normal style set to automatically update.
Format | Style, the style you are using (probably Normal) should be
highlighted in left column, click Modify, uncheck "automatically update", do
check "add to template", OK out of dialogs.
See here for a fuller explanation of why:
http://word.mvps.org/FAQs/Formatting/WholeDocumentReformatted.htm
(hit refresh a few times in Safari, or use a different browser)
Is this Word 2004? I think someone messed up and borrowed us trouble from
WinWord, that issue didn't use to show up here.