I just got completely tripped up by this...
A category with a particular quiz as its parent. Is this a new thing? I can't find any reference in the documentation to it.
It tripped us up because, of course, it's only in the list of categories if you access that list via the parent question.
A related question... when editing a question, what's the purpose of the 'Use this category' tick box? All it seems to do is to grey out the category drop-down.