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Josh K.First, I dont think you have enough empirical evidence/data to make that statement. This is one card, it happens to be graded by sgc. That doesnt mean that all misgraded overgraded cards by sgc will sell based on the cards value and not the value of the slab.
That said, assuming the truth of your stmt, I can offer you the most likely explanation:
The psa registry - people blindly pay for the psa number b/c their average grade will rise even if the card itself is a POS.
I think, as a general rule, collectors who are not interested in the registry or how their cards compare to everyone else will look at cards graded by companies other than psa and tend to focus on the card and not the number. Bottom line is you dont have the set registry followers bidding on an overgraded sgc card b/c it does nothing for them.