In a PPO health insurance plan, you get preferred rates for providers in the network.
If you go to out of network providers, the policy will pay a percentage of the usual, customary and reasonable (UCR) rate for the service.
The out of network provider, however, is not obligated to accept this UCR as the full amount due and can bill you for whatever they feel they can get.
For example, if the UCR is $100 and the the insurance company will pay 80% ($80), you might expect to pay the other 20% ($20). But, if the provider wants to, they can claim their fee is $150. The insurance company is still paying $80 and you would be billed for the balance of $70.