Anyone who has had an involved relationship with the US medical care system is likely to come to the conclusion that sometimes they just make their bills up, either to increase revenue or because their record-keeping is so chaotic.
If you doubt this, consider an analogy. Suppose you took your car in for major engine repair, it was in the shop for a week, and you paid the hefty bill. Now, suppose 4 months later you got another bill from a "muffler specialist" or a "catalytic converter specialist" for $ 300, with a code saying that they worked on your car while it was in the shop, but no indication as to what they actually did (except, maybe, look at your muffler or catalytic converter). Would you consider it legit? Would you assume you are being gouged? Would you pay? (They'll take you to court if you don't.)
In my experience, the medical version of this happens every time I have a family member in a US hospital. Not occasionally, not once in a blue moon, but every time. This is one reason why you never know how much a procedure is going to cost; you don't know what bills are going to show up months later.