The Japanese copyright laws must be significantly different from those in the West if they can get away with making substantive changes without the author's permission.
The English language version that was published by the author Ben Hills contained a number of errors. ( A number of quite obvious ones that even I as an Australian with a modicum of knowledge about Japanese royals could spot. A native Japanese with a lot of knowledge would have found many more) With the original Japanese publisher that was going to publish the book ( Kodansha) Ben Hills agreed to the correction of the mistakes, this then is where the Japanese version will be different from what was published in English.
Mistakes in the book include:
Ben Hills wrote that Masako was 7 months pregnant when she lost her first baby in Jan 2000, infact she was only a few weeks pregnant and the official pregnancy announcement hadn't even gone out, so she hadn't reached the 12 weeks stage.
He wrote that Akihito and Michiko were forced to wait more than 2 years by the IHA to marry as Michiko was a commoner. Infact they had known each other less than 2 years from when they met ( Aug 1957) to when they married ( April 1959) They met at a tennis party ( one of many social events organised by the IHA courtiers for Akihito to meet suitable young women. Being a commoner wasn't a problem as the IHA had already approved of her, if they hadn't she never would have been invited.
He incorrectly identified various Imperial family members and their relationship to Naruhito. Incorrectly made the statement that Prince Takamado ( who was a cousin of the Emperor's and supportive of Masako) that this was because he was the first Imperial Family member to be educated in the west, infact both his older brothers had studied in respectively the UK and Australia. Emperor Hirohito's brother Prince Chichibu studied at Oxford in the 1920's.
One error that I can't claim to have picked out, but rather the one the IHA press office issued a statement over was Ben Hills assertion that the Imperial Family members' activities were bland and they never would be seen in a controversial area. He gave the example of Diana, Princess of Wales being involved in the Leprosy Mission, and said that the Japanese royals wouldn't be involved in such a controversial patronage. The IHA press office pointed out rather strongly ( the letter is on the official site) that the Emperor and Empress had visited a number of Leprosy communities ( including one in Australia during an official visit there. One in the eye for Ben Hills' research skills!)
So as much as it is tantalising to see a sinister reason for the Japanese version to be different, it had to be to correct the mistakes and therefore there is a need to rewrite what was written in the English version.
As I wrote before, Ben Hills approved the correction of his mistakes with the first Japanese publisher, he would have the same agreement with this other publisher.