Likely a mistake and was pushed to patient responsibility rather than insurance writeoff. It happens occasionally in the billing system. Call the hospital billing office and ask them why you are being asked to pay an amount that is an insurance contractual writeoff. If they won’t budge ask them to send the remittance advice they received to you as your explanation of benefits (the insurance advice you received) shows something different. It is possible they also misapplied your payments. Just call them and talk with them about it. Now billing offices try to be reasonable.