I'm not 100% sure, but I think it has something to do with the volute where the neck joins the headstock. On older Les Paul's (by which I mean vintage ones) the volute is much less bulky, which some players prefer the feel of, but it makes that area weaker, meaning that it is more likely to break at that point if it takes a whack. I don't think it would be a problem in everyday use, though.