CoreLocation trueHeading not accurate

I found iOS CoreLocation trueHeading not accurate. Accutally, it was off a lot. When I point the iPhone to north, it reported a heading of 250 degree. Why?

Here is my code to setup location Manager:

        locationManager.delegate = self
        locationManager.desiredAccuracy = kCLLocationAccuracyBest
        locationManager.requestWhenInUseAuthorization()
        locationManager.startUpdatingLocation()
        locationManager.startUpdatingHeading()

Here is my code to retrieve the heading:

func locationManager(_ manager: CLLocationManager,
                         didUpdateHeading newHeading: CLHeading){
        let heading = newHeading.trueHeading;
        print(heading)
        radarView.heading = Float(heading);
        radarView.setNeedsDisplay();
    }