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I think the registry sets is an important factor in the price differences. NGC allows both in their registry sets while PCGS only allows their own which creates increased demand for PCGS slabs from those who are doing a PCGS registry set and don't want to deal with crossovers.
I agree, I do think thats definitely part of it. Theres some exceptions to this as well but for the most part the highest graded PCGS coins almost always have a lower population than the NGC counter part. Sometimes NGC even has a grade higher than PCGS does as a top grade which I believe is the other major factor driving the premium. There could be more too it but I think what you mentioned and the PCGS coins generally have a lower population at those levels are the major reasons for it