Nice film to cite in your description, Bleddyn!
What you're noticing is called "spotlighting" (or "hot spotting"), and it's inherent to the RPTV concept (though the extent of the phenomenon varies greatly among all the sets out there). You are seeing the point where the three CRTs focus most of their light. Some of this can be reduced by minimizing internal reflections within the chassis. But you can't do away with it entirely.