Medical billing in the USA is utterly ridiculous.

You get a statement that says, bascially:

We billed $725 to your insurance.

Your insurance company said "nuh-uh", you can only charge us $50 for that.

Then the insurance company pays $40.

And then they still bill you $10.

So... $675 just vanished into the ether, but it's absolutely critical that I pay $10.

Because, we absolutely could not have the abomination of me getting something for free, just because I already paid for it, could we?