Oxycontin is labeled for use twice a day, so I imagine that's how insurance companies would reimburse. If they are not lasting the full 12 hours, you need to up the dose, or use a breakthrough dose to get you through those last few hours. Personally, I don't like Oxycontin for this very reason.
And no, pain clinics and pain docs don't usually understand the details of medicare, or your insurance plan. They write the Rx, but have no role in dispensing medication.