There are two reasons why you can get a bill, compared to your previous plan:
Your dentist was in-network with your previous plan, but not Bento.
This means your dentist has not agreed to the price limits for service that Bento in-network providers have. Keep in mind, you are getting a bill because your dentists rates are both higher than Bentos maximum, and they chose to not write off any difference.
Your dentist was out-of-network with your previous plan, they used to write-off the difference, but have stopped now you have Bento.
Because your dentist values your relationship, often times even though an office is out-of-network with an insurance company they will still not bill you for the difference between their office fees, and the rates your plan pays. If this is the case, you should ask your office why they have stopped doing so.
The procedures performed were more complicated or intensive than you previously have had.
Some procedures, such as fillings, root canals, crowns, and even some special exams have an out-of-pocket cost with most plans.