I thought about it for a while, and figured out that the concept of female Ninja in the media outdates the concept of female Samurai by decades (not to say both didn't exist long ago, but I'm thinking in terms of movies/tv/comics). Since Ninja is little more than Samurai with slightly different morality, and Chiaki was always on the edge of that morality anyway, it made sense. Heck, in modern-day Parodyverse, Chiaki is always on the edge of becoming Ninja - all she has to do is abandon her personal code of conduct and kill and maim for hire.