I would think you can find all types of examples of broken impacted bullets, just by googling impacted bullets you see a few examples.Not exactly like your example but what it hits would change the amount of damage to bullet itself. If it hit something soft then hard it wouldn't deform as much as if it hit hard object only, I don't want to get graphic in description but I think you will understand what I refer to.Also the lead could have had impurities in it that could have caused it to break apart upon impact.