According to lauren faust as she said in devianart,somewhere something like this
''When Nightmare moon was defeated, luna had just started regaining her good will and powers, which are represented by her mane, so during luna eclipse, enough time has paseed and now she posesses all the power she had before becoming Nightmare Moon''
Also, Rarity and Nightmare Moon and luna share the same voice actor, or i think so

Or else, we can say this <img src="https://i.kym-cdn.com/photos/images/original/000/343/442/ff5"