Although Japan normally uses 100V supplies for most of their lighting equipment, I have seen that some older fluorescent ballasts sold in Japan are rated for 110V supplies only while some older HID ballasts sold in Japan have 110V taps in addition to having the regular 100V taps. Why is this so?

Here is an example of an older Japanese market preheat fluorescent circline fixture that has a 110V 60Hz ballast in it rather than a 100V ballast:

https://www.jauce.com/auction/t1100292976

Here is an example of an older Japanese market HID ballast that has a 110V tap in addition to having a standard 100V tap:

https://jp.mercari.com/item/m55198647840

It's just a guess, but immediately after that war, Japan would have had a lot of help rebuilding from the US, so they could have been using US built electricity generators with US specs.