So, why the difference?
Why don't I write all my books in the same language style?
In short, it's my characters.
As I write their stories, some characters tell me they wouldn't use that word so I clean up the language. Yes, I hear voices. Others, like my hero in a book currently with an editor, have no qualms about using the F-word frequently. The hero in the book I'm writing now speaks with less spice.
If I choose not to listen to my character's directions, he or she wakes me up at night. Or preys on my mind during the day long after I've moved onto another chapter.
Not all authors think the way I do. And that's okay. They don't have to deal with how my mind works. I do. So one time you might read a book of mine with little swearing and other times the language will make your hair curl. It's my characters' fault.