Adam writes:

> Ok, so the reason is that the definition of a variable pitch face is
> whether it is, or inherits from, the face variable-pitch.
>
> I just used customize to set the font of gnus-header-name.

So my fix is to change this line in the custom-set-faces call in my
~/.emacs.d/init.el:

     '(gnus-header-name ((t (:foreground "maroon" :family "DejaVu Sans Condensed"))))

to:

     '(gnus-header-name ((t (:foreground "maroon" :inherit variable-pitch :family "DejaVu Sans Condensed"))))

And Bob's my uncle again: