Did you discuss with your insurance company the fact that the dentist had previously and subsequently billed them for services performed under the same policy and number? The requirement in their contract with the in-network provider for timely submission of a bill may also include a provision that if they miss the filing deadline they cannot pursue you for the money the insurance company would have reimbursed.
Your contract with the dentist may include provisions for late fees, interest, and collections fees. That would explain a higher amount. You can ask the collection agency to verify the amount claimed, but verification requirements are slight and it sounds like you're past the 30 day window from their first contact with you such that verification would be mandatory under federal law (the FDCPA).

