Mikiethebull
High Hand
I've had the exact same experience. "Unused" brown box casino KEMS that came in silly resealed plastic and were bowed and warped. I paid decent money for them, too.
Perhaps too much time has now passed to find pre-USPC KEMs in good conditions. Or perhaps there really is no difference and it's just luck of the draw if you happen to get a deck from a good lot. It's just odd to me that some KEMs bow and others don't.