What's the difference? A question about dental nurses.

I'm interested in the answer to a simple question.

What's the difference between a dental nurse who is assisting a dentist generating £20,000 per month of gross production.......

........and a dental nurse assisting a dentist generating £60,000 per month of gross production?

It's a serious question - and so I'd like to avoid flippant answers.

Is there a difference?

If so - what is it?