It's hard to say for sure, but it could be a combination of the following:
1. The number of cards brought into the show in the 70s was a bit less than 75.
2. While all of those cards looked high grade, some of them had minor flaws which would knock down the technical grade (eg. a minor surface wrinkle), assuming they were graded by a third party grader.
3. Only a small amount of those high grade cards from the 70s card show were ever sent into PSA, SGC, or BVG.
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