User blog comment:God of nothing/Shiba clan logic/@comment-8891571-20130429044719/@comment-4903773-20130513234109

It could be that Tite Kubo didn't come up with the idea of making Isshin a Shinigami from the beginning, but only thought of it later. In a manga like bleach there's bound to be inconsistencies, but I'm pretty sure Kubo will make up a good excuse to explain all of this later.