This picture has been rated PG-13 for Western Violence

I'm a movie nut, as well as a movie snob. I've been spending some time trying to work my way through all of the movies I should have seen by now, but hadn't. This week's was "Once Upon a Time In the West". I'm not much of a western fan and I don't really like Charles Bronson, so it took me a while to get around to watching this one. It's supposedly a classic. I'm about half way into it right now, and it might be. The thing that struck me as funny was the parental guidance warning at the beginning of the movie. It's rated PG-13. What struck me as weird was the statement underneath the rating. "This picture has been rated PG-13 for Western Violence and Brief Sensuality". I've also seen "Science Fiction Violence" and "Cartoon Violence". Does this really make a difference, what type of violence is depicted? Watching cowboy's shoot one another is somehow different from watching astronauts shoot at one another?