I'm betting everyone gets basically the same vanilla app and then the interface you're using generates a property list (similar to XML) that provides the configuration that the app uses to figure out how to lay out elements. The app simply copies this property list on launch. They mention that you can run your app within one that they will have on the store so I'm assuming that's how they're doing that as well.

This is one of the ways that the iTunes Music Store/app store work to keep content dynamic. Notice that when you're on a poor connection and you open the app store that the icons and names at the bottom of the screen take a while to show up. This is because the app is just a shell and it's loading its configuration over the network.