What cosmic laws and universal truths dictate what these genres of fiction are? Oh, that's right, absolutely none whatsoever. They're just labels that some guys invented and slapped on things that suited their classification. They are entirely arbitrary, the conditions for said classification were chosen by a person according to their own bias at some point, and only became acknowledged when people started to mindlessly adhere to them. All they do is strangle creativity and shunt stories into particular directions. They're meaningless.
It's the same with music. 'Stoner rock,' 'acid rock', 'arena rock', 'bandanna thrash' -- they're just things that people slapped onto certain kinds of music for the SAKE OF CONVENIENCE more than anything else. It's more convenient to accurately describe the kind of music you like as opposed to citing specific bands that someone might be entirely unfamiliar with.
But a decent artist is never going to feel constrained by these arbitrary labels and allow their work to abide by someone else's rules of classification. It's the brainlet's approach to art to even use any of these genres and subgenres for anything beyond shorthand for the sake of description. Again, it doesn't mean anything and it's not worth getting bent out of shape over.