In my experience working for manufacturers this kind of thing is usually a characteristic of the actual piano that was sampled. Also, the sample editing and processing required to get raw samples ready for prime time can have an effect on the sound that might yield results similar to what you describe. Sometimes harsh characteristics (harmonics, partials, etc) on certain notes/ranges need to be dampened and rolled off, etc. These steps *need* to be done or the samples would sound INSANE upon playback. The risk of fixing harsh partials is creating too much of a timbral difference between adjacent notes/ranges. Those of us who make a living in sound design drive ourselves mad trying to strike an optimal balance between smoothing out the sound while not going too far and creating unnatural transitions. Based on my experience and that of most PX customers I've observed I'd say that Casio did an admirable job tackling this tricky issue.