I guess the yamaha has dedicated DSP, it might also have some analog circuitry as part of the signal path. Both of these will change the quality of the sound coming out, potentially for the better. Also for some reason sometimes downsampling to 12bit gives a pleasing character as can be heard on some of the hardware samplers from the 90s. As for the sounds actually on the keyboard, its possible they were recorded from a different piano, with different mic placement and a different mic pre amp to the ableton sounds, all of which could potentially lead to a nicer sound being heard.