This is are just my theories.
They're often funnier because they're short stories. The theme/main plot can be funny without getting old, not as many jokes have to be made to keep it consistently humorous.
Also, perhaps more importantly they're not quite as massively commercialized, and all they have to do to sell is be hot. The rest of the stories' content they're free to work with, and they don't really have to worry as much about what they write in relation to their audience(as long as it doesn't detract from the piece's ability to arouse).
Finally, their target audience is older.