Layout problem with window base application

Question

Why do I have to do this in order to get the sub view of the application window in the right position?

- (void)applicationDidFinishLaunching:(UIApplication *)application {
    groupViewController = [[GroupMemberPickerViewController alloc]  
                                 initWithNibName:@"GroupMemberPickerViewController"
                                          bundle:[NSBundle mainBundle]];
    window.frame = [UIScreen mainScreen].applicationFrame;
    [window addSubview: groupViewController.view];
    [window makeKeyAndVisible];
}

I mean why do I have to set the window's frame to match the application's frame?. Because if I don't do this the sub view is located 5 px to the top. I just created a new window based project and add the following code. I did not touch any xib file apart from the sub view that I'm setting, but I did not touch nothing related to layout and position.