The UIVIewController knows about the two views. So it can query the master view for the selection. And it can tell the master view to change that selection.
But the detail view can't know about the master view. And it can't know about it's controller. So you use the data-source approach for the detail view to get the selection from the controller. And the delegate approach for the detail view to inform the controller when the user has swiped to another account page.