I am not an expert, but as someone who has occasionally paid cash for medical procedures, I know one way it happens: deliberately mis-classifying procedures. You've probably seen doctors check off a few things in a tiny print grid that you then hand to the receptionist to pay for your appointment. Many of the things on that are judgement calls or close enough to each other that anyone not in the room could never tell which it really was.

For example, if you go to your eye doctor, they can bill you for a standard visit, a refraction (getting your prescription), an eye health check, a super-duper health check because they found something and needed to look at it more, an eye health check with dilation, etc.

I have gone to eye doctors who will do everything and then ask at the end of the appointment what my insurance situation was. If I'm on insurance, bang, they charge me for all of it. If not, they just charge me for the refraction and forget about the rest.