David is "different" from the norm in two ways. Long before he has had any knowledge of a city, he has recurring dreams about a city that is beautiful and "fascinating," and he also befriends Sophie despite the fact that she possesses six toes, promising to keep her deformity secret even though doing so is strictly forbidden.
1. From David's narration of his dream, "it was a beautiful, fascinating place" with very strange-looking buildings; it is unlike any place he has ever seen. Also, the
...traffic in the streets was strange, carts running with no horses to pull them; and sometimes there were things in the sky, shiny fish-shaped things that certainly were not birds.
David wonders if he dreams of a city that the Old People lived in before the Tribulation.
2. The other way David is different from those of his society occurs in his reaction to Sophie's foot when he extricates it from where it has become wedged in the ruts of the bank down which he and Sophie slide. Despite one of the post-apocalyptic prohibitions that hangs for all to see as they come through the back door of his home--"WATCH THOU FOR THE MUTANT!"--David promises not to tell anyone about Sophie's sixth toe on her foot, which he discovers when he pulls off her shoe.
Sophie's mother is very worried because she knows that any mutation is strictly forbidden. But David assures her that he will keep Sophie's secret; morever, in making such a promise, he acts in defiance of his society.