It cracks me up that you can see her roots growing in here. Stars ARE just like us. It also seems, from this angle, that she's basically just wearing a tube top.
Okay, now I am kind of scared, because you really aren't supposed to be wearing white tie on your spine. Nor do you want a butt bow to migrate up to your bra-line. In short: there's very little to justify this, other perhaps than if some jokester at the premiere snuck up behind her and drew a face on her shoulder blades, so that this bow became a proper bow tie.
And yet from the front, it's kind of sleek and summery. Maybe she can just crab shuffle her way through the party with her back to the room the entire time. Desperate times.