Who owns copies of Spiderman 1? I think it is mostly a book that collectors own, rather than speculators. As a collector, most don't worry about a small decrease in value so their books will stay in their collections and off the market.
Who owns books like Hulk 181, WWBN 32 and NM98? In these cases, I believe a lot of them are owned by speculators and investors. For these people, a 20% downward movement may cause them to sell, adding more books to an already weakening market , creating buying opportunities for some.
With prices as they are today, I'm not buying anything for the longterm, just books I can quickly flip for a few bucks.