I've had a fair number of medical bills since my car was read-ended at the start of June last year (2017). Every month my insurance company sends me an Explanation of Benefits (EoB), and I've been reading them. They are weird.
For example, last year I spent a fair amount of time seeing doctors and physical therapists, and over the busiest six months of this, the local medical empire billed a total of $21,779.25. But of that, my insurance company approved only $4,756.59. They paid $3,375.76 and I paid $1505.77. Notice that their share and mine together add up to $4,881.53, which is about $125 more than the approved amount. What was that, a tip?
On the other hand, one of my physical therapy visits was billed at $780.00, but the insurance approved only $121.94, and the total amount actually paid ($40 by me, the rest by insurance) came to $120.30, which is $1.14 short of the approved amount. There's no explanation of this that I can find in the Explanation of Benefits.
The biggest difference, of course, is between the amount billed and the amount approved. I suspect nobody actually pays full list price, even the uninsured. It may be there just to make patients think they're getting a bargain.
One more oddity: Several times recently I've received checks for modest amounts of money from the aforementioned healthcare empire. The accompanying statements indicate they are refunds, but don't say for what. I've have no idea, but that doesn't stop me from depositing them.