When is a page not a page? % if not page % fails unexpectedly

In trying to solve the problem of two titles appearing on a taxonomy/term/n page, I've discovered that the Twig statement {% if not page %} doesn't behave as I expect.

Does anyone understand why the D8 core taxonomy.module determines that a url like taxonomy/term/n is not a page? The functions involved in taxonomy.module seem to be
function template_preprocess_taxonomy_term(&$variables) together with function taxonomy_term_is_page(Term $term).

To debug this with a a visible indication, I copied taxonomy-term.html.twig into my Bartik subtheme templates folder, and added this;

{% if not page %}
  <DIV style="color:#c00;">[NOT A PAGE] TEMPLATE taxonomy-term.html.twig</DIV>
  {% endif %}
  {% if page %}
  <DIV style="color:#c00;">[THIS IS A PAGE] TEMPLATE taxonomy-term.html.twig</DIV>
  {% endif %}

I still haven't found any instance when "if page" is true.