Spot on.

There is a chance that some of the "below average" are in fact mediocre but treated as below. Many times, when you talk to them, you realize that they know more than they let show, but that they know they would have an easier time if they pretend to fit the stereotype.

Work is already difficult, so having to endure extra obstacles and defending your right to be there in addition to do the actual work might be taxing. So people develop their own strategies to fit in. Can't be blamed for that.

No one should be asked to demostrate they have a right to be there, just do the job.