Assuming Sudclavier has really used a good recording quality, the difference could in part be due to better effects (as Bachus says), but it could also be a different degree of how the samples are compressed. In some forums users also attribute such differences to different quality of the digital-analogue converters. I remember a magazine review in which the differences in sound between Juno Stage and GW8 (despite largely identical waveROM) were explained that way.