I think it's to represent time passing and her getting over the things which held her back when Arthas was alive. She hasn't really had any goals but to kill Arthas since her resurrection... her entire story has put her in a state of confusion between LK and the current world. Her dialogue during ICC explains a lot. Blizzard has said the same. They haven't touched her in a while so there's definitely a lot we've missed that she's been through. Let it play out. Wait and see what happens.