What makes OON claims so hard to process?

For almost 2 years, I’ve submitted monthly superbills to insurance for the same therapist, with same CPT, diagnosis, and charge. Basically, the same bill every month, just different dates. My insurance has stayed the same, too. Same insurer, same plan.

It took me a while to catch it, but I noticed that the allowed amounts on the EOBs changed seemingly randomly. I understand that the allowed amount is the usual and customary rate for the service,, but I didn’t understand how this rate could vary from month to month, swinging back and forth from 100% allowed to about 68% allowed.

I followed up with HR, and they followed up with insurance and determined that this variance was incorrect. All claims were reprocessed.

And since then, still, from month to month the allowed amount varies, so I email HR every month for their help in getting insurance to reprocess the claim. HR finally forwarded me an email saying the insurance supervisor was giving the claims examiners training in these claims, but literally the next claim I submitted was processed with the same error.

I’m sincerely curious to know from people with experience on the claims side: what would make it hard to process these OON claims consistently? I assume there must be some confusion in the rules for the claims or how they are applied. Any idea what could cause that confusion?