Maybe it has something to do with the concept of "fat over lean" This is an oil painting term that refers to the drying times of various paints. Fat paint is paint as it comes straight from the tube. Adding oil to it makes it "fatter", thereby increasing its drying time. To reduce the risk of having a painting crack, fat paint is applied over lean paint - paint that has been thinned. Lean paint dries before the fat paint does. If you paint lean over fat, the top layer of paint dries before the underlying layer does. Consequently, as the underlying layer of paint dries, it actually pulls the top layer of paint apart.
I've bought JLs that were cracked when I got them. The green Super 70s Torino is one them. I've seen a few of these, and the white painted roof was cracked on all of them. I saw the same thing on the Show Rods Wildcat tonneau cover.