[CPA]

This might be a billing/coding issue. Since you are receiving treatment every three weeks, not every week, you are receiving a larger dose (more expensive per dose). When the insurance rates were calculated, they may have been calculated at the weekly dose rate.

You may have to talk to somebody relatively high-up at the insurance company to find this out (most of the first level customer service people will have probably no idea what you are talking about).

I could be completely wrong with this, but it is worth looking into...