Usual, Customary, and Reasonable Charge
Most insurance companies do not use your actual bills to calculate their payments. Companies have their own fee schedule, often known as usual, customary, and reasonable (UCR) charges. The UCR charges are typical amounts paid for everything from a doctor's visit to heart surgery.
For example if your doctor charges $1,000 for a gall bladder operation while most doctors in your area charge only $800, you will be billed for the $200 difference. This is in addition to the deductible and coinsurance you would be expected to pay. To avoid this additional cost, ask your doctor to accept your insurance company's payment as full payment. Or shop around to find a doctor who will. Otherwise, you will have to pay the difference.