Posted By:
NickThere are collectors breaking out high-grade SGC cards to send them to PSA for the purposes of sale, because they know PSA generally draws a higher price. This selective submission of high-end cards to PSA will also inflate PSA's percentages in high grades on certain sets.
Considering the population disparity on '33 Goudeys, I doubt it's applicable there to any great extent.
However, I think a more accurate picture of grade leniency could be gained by taking the pop reports and knocking out the cards that people will get graded in any condition - Ruth, Gehrig, Foxx, and a few other HOFers. If SGC is grading very few commons, it stands to reason that the average grade given by SGC will be lower - people have no problem sending in a P or F condition Ruth, but if a common is submitted in that condition, the submitter normally missed writing or a pinhole.