I would not call 15% underpayment by 60% customers to be " a modest amount" if a company routinely had to settled 60% of its account payable with a 15% shave those companies would go out of business very quickly
Also in the Hospital context there are other factors that drive up the costs. Keep in mind that about 70-80% of all US Hospitals are non-profit
Hospitals complain that Medicare and Medicaid underpay, but it's a modest amount, not something that explains the very high costs for other patients: https://www.aha.org/fact-sheets/2020-01-07-fact-sheet-underp...