I'm no leather expert, but I suspect, being an organic material, from different cows from different places and different family lines, that each piece of leather, are slightly different from each other at the cellular level. Some stronger than others, some weaker than others. Some able to take on a lot of tension stress without tearing , and some not being able to as well. Put on top of that the possible non consistent tanning/processing of the leather before it gets to the saddle factories, and you can expect that the leather can sometime not perform as well as one might expect, no matter what you do to it or how gentle you treat it.....
JMOs