Conceptually, at least the way I'm used to thinking about documents ... a heading is something followed by a paragraph or other text object; if I'm working with a program that gives me an overview of document structure, I expect to see headings in that overview.

The Table of Contents is just another text object, generated on the basis of headings. If I'm working with a program that gives me an overview of document structure, all I expect to see is the heading "Table of contents". The entries in the table of contents are _not_ headings, because they are not themselves followed by text.

I've just been skyped by the colleague in question, and now we notice that while the Navigation (still) includes the ToC entries, it does not include the ToC Heading - so in terms of navigation, the Navigation pane is failing to show part of the document structure! All very strange ...