Is it normal for a dentist to keep a credit from an insurance reimbursement?

If you had a dental procedure and your insurance covered it, but the dentist still made you pay out of pocket, shouldn't the dentist inform the patient immediatley once the insurance company reimburses them? Shouldn't the patient know that he/she has a credit? Is it normal for them to not tell you and just apply it to your next appointment without your knowledge?
--------------------
Usually dental insurances do not pay the whole thing. They pay part of it and then the patient pays the rest of the fee or a copayment. If you are unsure CALL YOUR INSURANCE COMPANY. The number is on your card. Call and speak to someone there. Ask them if they covered the whole dental procedure you had done. And then if they say they covered 100% then tell them that you also paid the doctor and the doctor didnt give you any money back. The insurance company needs to know if the doctor got paid 2 times (once by you and them) if he shouldnt have. They can help you. Most likely the insurance paid about 80% and you had to pay the balance.
Source