They have a 100v grid but the frequencies are split between 50Hz & 60Hz meaning depending on which part you live in, it may operate in 50Hz while the other is 60Hz despite the whole country having 100v as their main voltage. For some reason, they can’t decide on what to stick with since most countries choose one or the other.
The voltage and frequency are independent of each other.
A quick web search revealed this in the first result: https://www.electricaltechnology.org/2025/02/japan-50hz-60hz.html