I'm not doubting what you felt, but I'm a bit puzzled, if the air pressure is the same how would one butyl tube feel softer over another? I have experienced that sensation going from Butyl to Latex, I have felt a sluggish sensation going from ultralight to heavy Slime filled tubes, maybe that's what you felt? The weight of the cheaper Walmart tube being sluggish? But never what you experienced from butyl to butyl in regards to softness of the ride. If anything I would have thought the thinner tube would be more comfortable due to it's elasticity vs a thicker tube, but again I doubt you could feel anything except the weight. Maybe your the type of person that can feel a pea under your mattress, I don't know.

Maybe someone else can throw some light on this.