When you think of Saber, you think of Aturia. However, Saber Extra was supposed to be Joan of Arc, wasn't she? It's not really Aturia, but she is in the class of Saber. Saber Alter is dark Aturia, and Saber Lily is just Aturia in a different costume.
So I'm guessing this is either: A) A mistake on the artist's part. B) Intentional for the class/character-design and these are just outfits he made differently from the norm.
Or if I am to assume correctly, this was supposed to be 100% Joan of Arc. Her without armor in casual attire, her with her battle armor, Dark Joan of Arc, and Joan of Arc in a different costume. These are all original styles of the costumes, after all.
I think Joan of Arc needs her own tag. People will get too confused between the two, otherwise. Also, some people may take more pride in making art exclusively for Joan of Arc, and might cause a ruckus if their intentions are misconstrued. Every character deserves their own fan base, after all.