Yup, because you can actually buy the German version of the manga in Japan. (The covers are the German ones as well - Costa Caspary is the translator.) Thus the German Manga versions get their own pages on amazon.jp.
To find the Japanese Manga you have to search: "
キングダムハーツ" because amazon.jp is - you know - Japanese.
Thus the Japanese versions of Manga have their own pages like:
Amazon.co.jpF ƒLƒ“ƒOƒ_ƒ€ ƒn[ƒc 358/2Days(3) (ƒKƒ“ƒKƒ“ƒRƒ~ƒbƒNƒX): “V–ì ƒVƒ: –{
Amazon.co.jpF ƒLƒ“ƒOƒ_ƒ€ƒn[ƒc 358/2Days 1 (ƒKƒ“ƒKƒ“ƒRƒ~ƒbƒNƒX): “V–ì ƒVƒ: –{
And so on... These pages list Square Enix as publisher and Japanese as language.
There isn't a page for the Japanese version of KHII-6...
(See for yourself.)
I also remember Egmont announcing the KHII-6 a couple of times but the release was always delayed because the volume hasn't been released in Japan yet. I don't know why they announced it in the first place...
Edit:
Ironically there exists a page for KHII-6 on amzon.de listing April 2012 as release date. I guess someone saw this and created a KHII-6 page an amazon.jp
It's like Sephiroth0812 said: the Manga ist still listed as "Release Date in Japan unspecified" on Egmonts homepage.