The fundamental issue here is that Document Map guesses and displays what it
thinks is probably, more-or-less, kind of, a map of the document. If Word
finds a paragraph that smells like a heading, Document Map will change the
outline level of the paragraph and therefore display it in the Document Map.
A short paragraph, in bold, in a larger font than normal will almost always
end up being displayed in Document Map, because Word guesses that it might
be a heading.

As a precaution (though not a solution) do Tools Autocorrect Options. On
the Autoformat as you Type dialog, un-tick "Define styles based on your
formatting".

Actually, there are some solutions listed here, to fix this after the fact:

http://daiya.mvps.org/docmap.htm