I think you misunderstood him. Its not that they have people who possess the ability to absolutely identify the correct grade of, say, a coin. These people take a look at each coin that passes through their doors and compare their “correct” grade with that given by the grader actually assigned to each respective coin.

What happens is two or three people grade the item. If the first two people give it the same grade that is the final grade and both are considered to have graded it correctly. If they don't give it the same grade it goes to a third grader. If the third grader matches one of the first two then that is the grade and the third grader and the one he matched are considered to have gotten it correct and the other grader is charged with the error. If the third grader does not match one of the other two they take the average. So if one grader assigns a 61 and a second grader assigns a 64 and the third a 65 the average is 63 (I assume they round that). Therefor the grade is 63 and all 3 graders are considered to have gotten it wrong.