Actually the problem started back in the 1890s and even after the war it continued. The allied occupation (well actually the US occupied the entire country post war) did not change that. Outlet voltage is still 100V vs the North American 120V. Since some Japanese domestic electronics can have problems at 120, 120 to 100 V stepdown transformers are still sold.
This article is a good summary of how and why it happened from a source in Japan.
https://www.japantimes.co.jp/news/2011/07/19/reference/japans-incompatible-power-grids/
