these companies make millions of ammo a year. No company has 100% efficiency, just because you havent heard so so brand never made any bad ammo doesn't mean it didnt happen. All companies have a certain percentage of defects in whatever they produce. Your sampling size is too small in comparison to the amount ammo they make to make that generalization unless you have access to data the rest of us dont. Point is the company admitted there mistake and took care of the customer.
Machines can go out of spec when make millions of bullets a year. Some times the wrong amount of powder is mixed in. Keep in mind every time you shoot, your having a explosion go off in your hand, there's always a chance of accidents.