All to do with the fully weighted piano action action of the X4 I'm afraid. (And this is an area that Roland ARE good at, together with the Fatar Grand Touch controllers)

No synth action keybed (including the excellent PA3, or G70 ones) can ever hope to do proper justice to a piano sound. Close? Yes....but they lack the "bounce", key travel, key length (PARTICULARLY on the Yamaha series - even the Motif's)

If you took your module to someone who had a fully weighted Yamaha keybed, I daresay it would sound nearly just as beautiful as it did on the Fantom X8...again it is ALL in the keybed...