Why is it that when looking at someone like Heritage Auctions and comparing the same coin that sold in an NGC holder vs a PCGS holder, the PCGS coin demands more money at sale.

I have Studied several modern coins that are listed on the Heritage Auction site and five identical coins with the same identical grade the PCGS coins are getting sold for quite a bit more money. I personally only use NGC and like them very much.

An occasional mistake but they are prompt to correct it. I just do not understand why this difference at market between what I consider probably two equal graders.