Difference between Japanese and Japanese (Japan) for browser language

I'm analyzing language (using the out-of-the-box Language dimension) use on one of our sites, and I'm seeing consistent differences reported for Japanese users based on whether their language is "Japanese" or "Japanese (Japan)". Invariably, "Japanese (Japan)" performs much worse than those where the language is reported as "Japanese". I'm only looking at traffic from within Japan, by the way.

Does anyone know the difference between those two values?

Thanks!

Edit:

I think I figured it out, and it has to do with mobile. Mobile traffic is primarily reported as "Japanese (Japan)", where desktop traffic is mostly reported as "Japanese".