Total illogic in category-subgenre-genre-keyword connection

Because the system is not set up to force each subgenre keyword to be pigeonholed under only one specific genre. At times IMDb staff have acted as if that is the way subgenre keywords are supposed to work, but they haven't explained the rationale behind that approach (i.e., why they deem it necessary), nor have they fully implemented that approach by forcing all subgenre keywords to be paired with only one specific genre.

Personally, I think the system should be flexible enough to allow subgenre keywords that wouldn't always be limited to only one specific genre. The most obvious example is "neo-noir," which should be an accepted subgenre keyword (yet it is not), and which should by definition never be paired with the Film Noir genre. I give other examples here.

p.s. I thought of another example: the keyword hip-hop-music (408 titles) is currently accepted as a subgenre keyword. That keyword could easily be paired with either the Music genre or the Musical genre.

p.p.s. And here is yet another example: the keyword alternate-history (456 titles) is accepted as a subgenre. Yet, by definition, a title that features alternate history does not qualify for the History genre. This is thus a good example of an officially recognized subgenre that should never be matched with its counterpart genre.

p.p.p.s. I thought of yet another example: the keyword fictional-war (1348 titles) is, for some reason, accepted as a subgenre. Yet the IMDb help guide for genres tells us with respect to the War genre, "for titles that portray fictional war, please submit it as a keyword only." 

Boy, has this post been waylaid (hijacked?) as it wanders off the point of my regional posting!

"How is it possible for a title to have an IMDb "subgenre" as a "keyword" and not also have the "genre" attached to that "subgenre-keyword" as a "genre" for that same title?"

IMDb has created 28 genres.  20 of these genre have been designated by IMDb as having 210 subgenres.

My concern is with those 210 subgenres that have been created by IMDb with a "genre" suffix.  In IMDb's listing of those subgenres, they all neatly fit under 20 genres.

I raised my question again: "How is it possible for a title to have an IMDb "subgenre" as a "keyword" and not also have the "genre" attached to that "subgenre-keyword" as a "genre" for that same title?"

I am asking a question of simple logic.

Please, do not meander off into other issues, either general or specific, that should have threads of their own.  Thank you.

P.S. I can list many other keywords like "military" that should, or should not, have "subgenre" status, but that is a posting for another time.

@bradley_kent 

A couple from a hundred-thousand-million examples: Shouldn't American Beauty (1999) also have Comedy as a genre?  Some Like It Hot (1959) also have Crime as a genre? 

My answer to both questions is "no." @Peter_pbn provides more detail by pointing out that a film with dark comedy elements is not necessarily a comedy film.