[jhowell]

When Amazon introduced the KF8 (aka .azw3) format in 2011, the rendering engine for that format was not added to the iOS app, perhaps due to Apple's insistence on apps using the built-in Safari rendering engine for HTML. Instead that app uses a similar format (AZK/KCR) created for viewing books using a web browser (Kindle Cloud Reader). This causes books in the iOS app to be formatted differently from the other Kindle apps/devices in some cases.

In 2015 Amazon introduced another new format, KFX (aka Enhanced Typesetting). KFX is not based on HTML and the same rendering software is present in all Kindle apps and devices updated since then, including the iOS app. So if you publish a book on Amazon and it shows "Enhanced Typesetting: Enabled", as most books will, then iOS users running the Kindle app will receive the book in KFX format and it will be displayed using the KFX rendering engine.

However, this may not be relevant to you at this point. KFX is a language-specific format and I don't believe that it supports Japanese yet. So if you are publishing a Japanese language book then it will not support Enhanced Typesetting and will still be using the older KF8 and AZK (on iOS) formats. In that case what you have been doing makes sense.


Added: I have seen hints that Amazon is working on improving Enhanced Typesetting support for Chinese and adding support for Japanese. I don't know when.