Just a theory, but because SPD and other sellers of refurb/recert and white label drives have to wait until drives are pulled from service or enough binned drives accumulate in the manufacturer's warehouses, they have a much more limited supply chain. They can't just go to their supplier and say "I want X number of drives at this price." This is reflected in their often limited inventory and price fluctuations. To be clear, I'm sure they determine what they're willing to pay. But there's a balance between having to pay more and profit less, and not having any to sell at all.
I think the above is illustrated by their recent offering of refurb/recert Seagate Mach.2 drives. It's been reported that Mach.2 drives have been found in some of their externals, which makes sense since they were likely binned at the factory. But enough refurb/recert drives, probably from returns didn't accumulate until recently.