A curve is a curve whether it's against the highest score in the group or an overall pass rate. The score on your cpa exam is independent of anyone else's score. Otherwise, they wouldn't publish pass rates because they'd always be the same, nor would trends occur as they do. It's not an arbitrary rate. That's what pretest questions are for. From the aicpa:
Is the CPA Examination scored on a curve?
The CPA Examination is NOT curved. Every candidate’s score is entirely
independent of other candidates’ Examination results.
The CPA Examination is a criterion-referenced examination which means that it rests upon pre-determined standards. Every candidate’s performance is measured against established standards to determine whether the candidate has demonstrated the level of knowledge and skills that is represented by the passing score. Every candidate is judged against the same standards, and every score is an independent result.