I've been routinely graded WORSE than the average student and believe this rather confidently. An impression I get from having had teachers yell at me in the hallway about wasted potential and other students cracking jokes about how my friends had "ruined me" after my grades fell off a cliff after making some new ones. Grading doesn't prevent inflated delusions of intellectual superiority if people believe grades are a matter of hard work rather than intelligence.
What drove a sense of inferiority for me was somebody getting better grades with seemingly less effort.
I understand your point and, as I am sure you know, life can definitely be unfair.
Do you think it's so unfair that 90% of students would feel the same way? It's not impossible but personally, I would be surprised by that. Which is the point. Experiences vary dramatically, especially if we broaden the statement internationally, so knowing who was asked what is just as important as the claim.
I don't see it as unfairness, college mostly gatekeeper business and academia and raw brainpower isn't what you should be selecting for. At least not in isolation.
I do agree the 90% claim seems dubious especially unsourced.
What drove a sense of inferiority for me was somebody getting better grades with seemingly less effort.