Hardly any sample-playback synths are true stereo. I doubt that the piano sound contains actual stero samples. Probably, the different sample-zones are panned differently.

There are reasons for using stero, however. Sounds use panning to place sounds and elements of sound in the stero field. Effects are also stereo, most notably chorus, stero pan, and phaser fx sound a lot thinner in mono.

regards
Knut Inge