I understand now. Something just didn't add up in my head. Even though the original ML is different, I couldn't see why the original FL would be so different, too. In reality, the FL wasn't the duke's daughter at all, but the impostor he had originally set up to pass herself off as his “perfect daughter.” Maybe I'm an idiot for only realizing this now.
Tsukihiro spends so much time gazing at the moon that the stars have gone on strike for more attention !