While it's true, the US does supply 120V±5%, appliances and equipment are operable on a much wider range.
For example, my laptop charger is rated for 100-240V, which is not uncommon.
For example, my laptop charger is rated for 100-240V, which is not uncommon.
Power supplies are rated for 100V because that's the voltage in Japan. Though the tolerance would probably be useful for running a really long US extension cord.