Well for one, in general anything that comes from overseas is just "automatically" more expensive (e.g. ALO Audio Rx Mk3 is $650 vs $920 in Japan). I've imported stuff directly before and it's not always import taxes (which usually is minimal). So I guess labour, bricks 'n mortar.
Secondly, specifically for the Mytek, I believe the distributor has specifically asked Mytek Digital to produce Japan-specific models for the 115V -> 100V PSU. SQ-wise, he told me that the local Japanese ones sound "warmer/involving". I have heard only the Japanese version and it is warm-sounding, but never heard the 115V version as a comparison. But not worth the $1400 price difference.
If mini 115V -> 100V stepdown converters exist, this could be cheap option. Then only have to worry if the 60Hz -> 50Hz difference matters or not.
Thats actually really interesting, that it gained warmth due to the PSU change. As of right now, I would never call the American version warm but neutral sounding all-over the spectrum. Did you decide on which one you are going to buy?