I wonder if it is really the material of the case that makes most of the difference here.
The inclination of the keyboards can make a big difference in feel, as well as thickness and materials of the keyboards' feet.
Something that can also make quite a difference is the sound: if a keyboard sounds harder then can also appear to be harder than it isn't.
I have never tried a KUL (not available in ISO AFAIK) but most electronics have cases of injection-moulded ABS, with a slight "sandblasted-like" texture.
ABS is an alloy (of Acrylene, Butadiene and polyStyrene) with many different formulations. I have build models and cosplay costumes out of sheet ABS - and that contains a higher amount of butadiene making it softer than most keyboard cases.
Pre-aluminium Apple keyboards and keyboards from Matias were made from polycarbonate, also injection-moulded. It is tougher than most ABS and can be moulded clear. (ABS can not, it would have to be AS or S to be clear).
I built the case for my Phantom out of old plastic ABS keyboards cases, with special solvent-based glues for plastics so that the parts get "solvent-welded" together. Somehow I did manage to make it quite hard and very loud compared to my bought keyboards. I think that is part because I made the plate sit quite tight, with supports all over the bottom and not just at the edges but most of all I suspect that the loudness is because I put rubber feet only at the back so as to keep the keyboard as low-profile as possible in the front. The painted front edge where it touches the hard desk is a bit worn.
