EDIT to add:
Most places have a base + bonus structure. You get your base salary, and you see patients, for each patient seen you generate 'RVUs' which is how your group/practice generates income ( by billing insurance companies ). Once you generate enough RVUs to cover your base salary, you start accumulating 'bonus' and that gets paid out down the line using whatever formula your employer uses. There is some variation to this but for the most part groups follow a similar scheme.
EDIT #2: This is US centric, i dont know how other countries do it.
That's the insurance companies' stance. The work you performed is this and so our agreed upon rate is this.
The 12 y/o say ‘no you stink’ and hangs up. Then you send the landscaper a letter saying ‘sorry your peer to peer was denied’
( I know this is exaggerating a bit and made to sound funny but it mostly works like that in healthcare )