Why does the traileer for The Chronicles of Narnia the voyage of the dawn treadrer show all four pevensies?

The shots of Peter and Susan in Narnian clothes are "not what they seem" according to a quote from one of the lead actors in the movie, and they have definitely not returned to Narnia. From the limited amount of evidence available, it seems most likely it is some sort of magical illusion caused by the Magician's Book in Coriakin's House.