I think that the "legs" attached to the humanoid part of her body count more as pedipalps than actual legs. Pedipalps are additional appendages spiders usually have around the mouth to help them eat food. Okayado noted that in the case of Arachnes those are there mostly to keep their partner close to them during the sexual act and he used to draw them a lot smaller, now their proportions on the manga (and the anime) are more similar to actual human legs.