I had a 'fun' experience along these lines with health insurance and medical bills a couple years ago. I can confirm that in our case at least, /every/ error we found was not in our favor, and took usually about an hour on the phone to get fixed.

The somewhat-less-malicious interpretation is that the companies have a strong incentive to detect + fix errors that cost them money. Meanwhile, consumers are a) non-centralized, uncoordinated, and often unaware of errors, and b) have no way to fix systemic issues that impact them. And the companies therefore have no /real/ incentive to fix systemic problems. It is literally more profitable to fix the bills of the few people who complain, as they still make money on the remainder who don't notice the errors in the first place.

(on edit; exactly what the other comment one subthread over said. :P )

decades of data?