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Jerry Hrechka Not all cards in PSA Set Registries are equal. Some cards have a higher "WEIGHT" which can range from 1 to 10. For example a common T206 would have a weight of 1, while the T206 Honus Wagner has a weight of 10. So a common PSA5 would have 5 x 1 = 5 Grade Points while a PSA2 Wagner card would have 2 X 10 = 20 Grade Points.
That's probably were the difference between your calculations and PSA's is.