The interwebs (wiki) says they pay 6.6 million people a year. So at 99.76% success rate, there's nearly 16,000 people with pay problems. That's assuming those metrics were run per person and not per transaction. Wiki says they do over 455 million transactions a year. With a 99.76% success rate that means they fuck up about a million transactions a year.
Alibis for 2- math in public and using wiki as a resource.