Korg SP-200 - Stretched Tuning Explained; About Stretched Tuning

Other functions 21
About stretched tuning. In order to produce the most natural resonance, PIANO
1 and PIANO 2 sounds use a “stretched tuning” that makes the notes of the lower
range slightly flatter than equal temperament, and the upper range slightly sharper.
This is how an acoustic piano is normally tuned by professional tuners.