User blog comment:Mataaitai/Volume 32/@comment-48285-20121230201451

I have a theory. Perhaps it isn't so much the drawing process as it is the writing. You gotta remember that he writes the story as well as draw it. He might need some time getting the story thought through to see if it's going in the direction he wants it to go. Maybe his original plans didn't quite fit and so he has to redo it.

This is all just speculation on my part, however. I admit that taking months to think through a story is pretty ridiculous but it might not be too much of a stretch. With regular series novels, the author releases them a year apart (on average) because it takes time to think of the story.