Hoe handles are not always straight, but if that is the bokken in question, it could very well have been when it was being used as a hoe and then steamed/bent and carved down when made into a bokken.
Have you ever seen a Japanese hoe that is that shape? That looks African. I tried to look for old Japanese hoe's but couldn't find any that weren't straight.
I hope no one checks my browsing searches for Japanese Hoe!!