According to PSA, there is never a (single) grader. Their website says (
http://www.psacard.com/Services/PSAGradingProcess/):
"Grader #1 will then enter his grade for the card in question (and for each card within the order until the order is completed if there is more than one card) and close the order on his screen. Once that is done and after redistribution of the order, Grader #2 will do the same – not knowing the opinion of the first grader on any of the cards within that order.
If their grades match in the computer, the card would then eventually reach a 3rd grader for verification of the grade. If the opinion of the first two graders does not match, that card will be assigned to a 3rd grader whose opinion is required to break the tie, so to speak. As with the first example, the card would still be assigned to another grader for verification (a 4th grader in the process) to make sure the grade is accurate and consistent with our standards."
Based on the above, three graders had to agree that it was a 10. I also assume that "3rd" and "4th" grader above are not in reference to 9 and 10 year olds.