为什么我使用Addsubview添加其他控制器的视图，该视图将向下移动20点？

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    UIViewController *controller = [[UIViewController alloc] init];
    controller.view.backgroundColor = [UIColor whiteColor];
    [window setRootViewController:controller];
    UIViewController *controller2 = [[UIViewController alloc] init];
    [controller.view addSubview:controller2.view];
    controller2.view.backgroundColor = [UIColor yellowColor];
    UIViewController *controller3 = [[UIViewController alloc] init];
    controller3.view.backgroundColor = [UIColor purpleColor];
    [controller2.view addSubview:controller3.view];
    [window makeKeyAndVisible];
    return YES;
}