Has anyone noticed that as you play a chromatic scale down from the top (or up from the bottom) of your digital piano that every few notes there is a distinct change in timbre? I can only guess that is due to the fact that most digital pianos do not have a sample of all 88 keys of an acoustic piano, it is a smaller subset with all the rest of the notes artificially transposed and filled in. I first noticed this when one of the higher d-flats on my roland F-100 sounds slightly less brilliant than the other notes. I made sure this was the case by turning off the touch sensitivity briefly and listening to the timbre of each note.
Acoustic pianos have subtle changes in timbre for each note as well of course, as well as having some tiny flaws in tuning (even the best tuners aren't perfect). I suppose digital pianos just accentuate this fact by having fewer sampled tones.