Seems like an easy way for manipulation. Grade a borderline 9/10 a 9, with internal marking from the original grader. PSA offers full market price for the 9, or maybe even slightly above. You sell it to PSA, then they just change the label to a 10.
Maybe it isn’t an inside job, but maybe all 9 bought by PSA automatically go through another QA check to see if it could bump to a 10.
Grading companies should not be allowed to sell their slabs. A monopoly company now controls the value of the card