i think thats where the whole 'journey the world and write a book" prophecy comes in...even though he wrote the original book, it seems like he felt that was insufficient.....thats the 'follows whats been said in the manga' explanation.

If you want a more realistic explanation, Kishi came up with this 2 weeks ago, so he had no idea it would turn out like this, or that jiraya would play such an important role 3 years later.