I agree this is one of the best threads we've ever discussed here on the forum. It shows once again that there are lots of variables in our sport & that one time a bullet can succeed & the next time it can fail, even if the animal is smaller than the one where it was successful, go figure!
Maybe it's the angle, maybe the point of impact, mostly I think it goes back to velocity, just my thoughts but I'll give my reason for it.
On this Buffalo hunt the first 2 shots were at 75 yds with a muzzle velocity of just over 1600 fps. Not sure of the impact velocity but you math majors can figure it pretty quick. Maybe, just maybe that had something to do with the success of those 2 shots, just my thoughts.
Going back to one of Max's replies earlier on using a 400 gr hard cast at the Bovine Bash, he said it blew up on impact at close range, I'm not really surprised that it did, that's asking a lot of any bullet that's not pure copper, etc. Again, here's another maybe, what if that 400 gr hard cast had been going 200 fps slower on impact, would it have preformed better, I don't know, you guys have a lot more experience on those big animals than I do, I have no experience on them but every bullet except for the big solids has it's limits.
An XTP at the muzzle might have blown up just like that hard cast but at 75 yds had slowed down to where it was within the design limits of that bullet.
All of this is just asking questions, but velocity plays a factor in how a bullet fails or succeeds & this latest thread has really been a good read, I've enjoyed it very much.

Dick