Actually, judging by the 8 visible leaves and the space behind her back (which one would expect to have a leaf as well) I'd say she has 9 leaves; Perhaps she is excluding her own tails because they're not "fluffy" enough to count?
Or I could be wrong and it could just be an error- Touhou Wiki quotes something along the lines of "Despite being able to use powerful black magic and is capable of speech, she only possesses the intellect of a human child" about her, so either way... it's adorable regardless :3