UIVIewController wrong position before rotate

Question

I have the starter point of my application this window (white background):

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    mainScreen = [[MainScreenController alloc] initWithNibName:@"MainScreenController" bundle:[NSBundle mainBundle]];

    // add it to the main window
    [window  addSubview:[mainScreen view]];

    // show view
    [window makeKeyAndVisible];

    NSLog(@"started");

    return YES;
}

And MainScreenController is an UIViewController with (blackBackground) created by the Interface Builder.

The application executes perfectly but the first time, the black view is moved up like the height of the status iphone pannel (leaving white rectangle at the bottom).

If I rotate the iPhone twice, the view is perfectly positioned in the right place, filling all the screen with the black background.

Any idea?

Thanks!