Too good to be true? Practices offering GPs up to 90% of billings

Searching GP job advertisements these days turns up dozens of offers for GPs to keep 80% of their billings or more — even up to 90% in rare cases.
Some involve a drop down to 75-80% of billings after two or three months but certainly not all of them.
Given the frequently cited ‘normal’ percentage of billings is 60-70%, it seems that GPs should be in a prime position to negotiate and earn more.
But is that the reality?