为什么itouch上的应用使用[[UIScreen mainScreen] bounds]初始化时，得到的不是全屏的高度

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    // Override point for customization after application launch.

    self.window.rootViewController = [[GlobalController getInstance] viewController]; 

    self.window.backgroundColor = [UIColor whiteColor];
    [self.window makeKeyAndVisible];
    return YES;
}

但在itouch 5上运行时，却发现实际的应用高度只有480。整个状态栏都下移了, 有什么办法实现应用的高度等于全屏的高度？