Earlier quoted context omitted.
There may not be. There seems to be something obsessive about metal fans to catalog and classify and document. I mean, look at the wiki page on subgenres: https://en.wikipedia.org/wiki/Heavy_metal_genres It fills me with awe and giggles.
I wouldn't take that list too seriously. Metal-archives genres tend to not delve into "styles" as much. But there are certainly many distinct sub-genres and one can't exactly argue that Black Sabbath, Dimmu Borgir and Cannibal Corpse sound similar. I agree that the community itself takes the genre thing way too far and it often ends up being pretty funny. Then again, electronic music probably gives metal a run for it…
Some people are in on the joke and some are serious. Sometimes, it's hard to tell the difference, but that's OK, because I like talking with both types of people. It's usually informative and/or entertaining. :)