If the rails are not parallel then all he is doing machining it parallel to the twist. The bed will still drop as it moves along with the twist. I think you will find i am right on this one assuming that is what his problem is? Place an object on the bed at one end and check with a height gauge. Move the bed along and reposition the height gauge on the object and you will see it is different. The gauge will need to be set up off the machine.