Maybe the previous owner had the truck rustproofed and he had them spray it before putting it in the truck. If it came from the factory with the bedliner, then maybe the owner had it taken out, sprayed, and then put back in (I've seen stranger things). Some of the rustproofing goop they use can be pretty slimy. It definitely didn't come from the factory like that. Factory bedliner is dropped in with no fasteners. The only thing holding anything in are eight (8) torx screws on the tailgate portion. That's it.
My 08 B4000 SE came standard with the factory bedliner. It is strictly dropped in with nothing underneath it from the factory...it's the standard finish.
Removing this thing is going to be a great pleasure. I'm waiting on an appointment to have a spray-in installed to replace it right now.