Well everybody is partly correct. The voltage in Japan is 100 volts. The eastern half of the island is 50 hertz and the western half is 60 hertz. Those are facts.
The rest is an educated guess. I assume it is designed to function properly on either 50 or 60 Hertz so they don't have to build 2 models for the Japanese market. Unless there is a fan or motor in it that is frequency dependent that is simple to do. They probably configure US models for 120 volts. It may be as simple as moving a few wires on the power transformer, which any compotent tech should be able to do. Hopefully it isn't a different transformer but that would be very unusual. I also assume that it would be better to have the unit configured for the proper voltage or it would not be labeled 100 volts.
The rest is an educated guess. I assume it is designed to function properly on either 50 or 60 Hertz so they don't have to build 2 models for the Japanese market. Unless there is a fan or motor in it that is frequency dependent that is simple to do. They probably configure US models for 120 volts. It may be as simple as moving a few wires on the power transformer, which any compotent tech should be able to do. Hopefully it isn't a different transformer but that would be very unusual. I also assume that it would be better to have the unit configured for the proper voltage or it would not be labeled 100 volts.