I'm not making an argument for or against XP's but I know that Lynn used them in Australia on Buffalo. The 300 gr bullet in a 44 magnum & he shot a lot of them. Many were shoulder shots so it's like Rey said, it must be that some of those bullets in some calibers are made with a tougher core/jacket than others in the same caliber.
When I toured the Hornady factory last year I was doumb founded by the sheer quantity of bullets they were turning out. I don't think anyone sells as many bullets as they do. They had a LOT of employees. You would have to see it to believe it.
I have a friend who is well known on many of the handgun forums who has hunted world wide & I know that some of the large animals he's taken were with Hornady bullets. Not sure which ones so I won't venture a guess on caliber but he might chime in here. Pretty sure one of them was a 400 gr slug, just a guess, perhaps a 325 gr.

Dick