updated 03:23 pm EDT, Thu October 3, 2013
Uncertainty over whether cause is hardware or software
The iPhone 5s' motion-sensing functions can be substantially inaccurate in some cases, tests reveal. Gyroscope readings, for instance, can be a few degrees off what's reported by an iPhone 5 or a spirit level. The compass in a sample 5s was between 8 and 10 degrees off an iPhone 5, and worse, the iOS Compass app is allegedly prone to freezing or wildly inaccurate readings on a 5s, both problems that may force users to kill the app and restart it. Either device may fail to show the same magnetic north as a real compass. The 5s' accelerometer is meanwhile said to display more latent motion, although that effect is difficulty to quantify.
It's uncertain whether the issue is related to badly-calibrated hardware or glitches in iOS 7. It is believed to be widespread, but affecting different people in different ways. A software-only flaw might be easy to fix, but hardware could force Apple to make changes to manufacturing and/or release some sort of recalibration patch for iOS. Some people say that they've had success exchanging faulty iPhones for new ones, but others complain that nothing changed with a new unit.