Jay, you reference this as though it is proven fact. How about making a chart of 100 or so items that have been sold both in Sig auctions and in Major Auctions (or in Weekly Auctions vs Thrice Weekly Auctions). I think you will find that this is a myth.
And of course, such a chart would not even begin to take into account that one site has hidden bidding.
Well, I did use the word probably, which doesn't mean proven. I do research both your history and theirs.
The Roberta card cost me about $300 with buyer's premium. You've never sold that particular card, but other cards in VG-F condition have gone for between $200-$400, so we are in the ballpark. This card had nice whimsy, but no Ginger.
The Top Hat card was first release and a nice portrait card of Fred and Ginger. They Rated condition as 6.5. It had some work done on the borders, so VG-G is a fair evaluation. I paid $1900 with BP. You only have lesser cards with prices from $240-1100. You do show this card, but only as part of the archives.
So, I think I made a fair assessment. I suspect the Top Hat card may have made your major though. In any case, the prices from Heritage were in your ballpark, as much as one can compare apples to oranges. Don't worry, you're still my go to auction site.