Dental insurance said they’d pay $775. I now get a bill saying they only covered $86.

Hey guys, I got my wisdom teeth removed in March. Out-of-network because I needed specialty. The insurance website says my out of network deductible is $1500.

My total bill was $3,953. My oral surgery says “We make every effort to contact your insurance company to ‘estimate’ what their payment will be. Our insurance coordinator will call you with the amount due prior to your treatment”.
When they called me, my bill was now around $3,100. So insurance “covered” $775.

3 months later, I get a bill in the mail stating that my insurance paid only $86.40, and I now owe the remaining $688.60.

What? Why?? Please help me understand this.