You could shoot 2 deer with the same load distance etc.... and one drops right there, and another, acts like nothing happened, runs off and tips over in 200yards.
the 7mm mag and 140ttsx is certainly lethal on mulies.
if a different load gives you more confidence, go with it, but I think, that's mostly just the randomness of hunting/shooting/killing big game animals.
I remember a buck my dad took with a 12ga slug at maybe 50yds, just stood there, my dad put I think 4 rounds into the deer, eventually it staggered off, died, when field dressing we found 4 rounds all right in the heart region, good tight group, crazy to think the deer didn't just fall over after the 1st shot, but that's hunting.
It was a big buck however...