IPhone 5/6 returns various CLHeading magnetHeading values

Question

The following code snippet displays different values ​​for the iPhone 5 and iPhone 6 in the exact same position. What could be the reason?

Two different iPhone 5s output almost the same values, but iPhone 6 displays arbitrary values, as if its link always changed. I also tried with another iPhone 6+, it has a similar behavior with iPhone 6. Is there something missing about the latest iPhone?

-(void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)newHeading { NSLog(@"heading: %lf" , newHeading.magneticHeading); }