Maybe because they keep changing their name? (9X, BA, VZ...)
Many years ago I was involved with administration of a campus system based on NET Centrex, feeding many key systems. When I got together a few procedures to reconcile our work orders and their equipment bills, I noticed that new equipment would be installed and billed, and often old equipment would be removed without the billing being stopped. I would calculate the amount we had overpaid (sometimes over many years) and my bosses would ask for the credit. NET did not want to pay for more than a few months back, but we insisted.
At a meeting with our NET rep, she asked, "Suppose we found an instance where we had not been billing you for a service for several years, would you want to pay for all of that mistake?" I answered, "Yes, of course, but only if you can prove that the underbilling was our fault." We never did find any instances of underbilling anyway.
We seemed to win that one, probably because any escalation would have resulted in bad publicity for NET, and more institutions checking their bills.