These are all great comments and correctly thought out. I also wonder if the phenomenon of dealers buying books at the top of an initial auction, assuming they can then either press and flip or just price up and put out for sale is another explanation for these price drops. Can't tell you how many times I've been outbid on a book at Heritage, just to see it pop up at a dealer a month later, marked 20-40% higher than the final auction price.
Long ago when I was a manager at a LCS, I would never have survived buying books like that. Even books that I thought were a premium and in demand. It assumes that there are somehow other buyers, unaware of the auctions that will pay higher prices. Or that the market is growing so fast that prices are increasing at an incredible rate. I believe these numbers contradict both theories. What we're seeing may well be the #2 and #3 bidders after the same book again at another auction. Without the #1 bidder, the price drops.