Some (or all) motion sensors in the iPhone 5s are screwed up. According to some investigation by Gizmodo, based on user complaints, the device's sensors (gyroscope, compass and accelerometer) are all out of whack—and that's putting it lightly. The device actually does a pretty poor job when put to the test against real-world objects. It fails against Apple's own iPhone 5, too.
Gizmodo's testing involved two iPhone 5s units with the latest version of iOS 7 installed, and then put those up against the iPhone 5 and real-world tools. In most cases, Apple's own built-in measuring tools were used—you know, the whole hardware/software synthesis. Turns out the iPhone 5s's sensor are off by quite a bit, while the iPhone 5 is spot on. Gizmodo surmises hardware could be the culprit, which means a simple software fix won't make the problem disappear.
Each sensor is tested one by one, and each ends up getting a failing grade. Many of them might not mean all that much to most users—when's the last time your mom opened up the compass app? But when playing games, as Gizmodo demonstrates, it can be a bit of an annoyance; it'll sure make Temple Run a bit of a challenge.
As of now, it's unclear whether hardware or software is to blame. What is clear, however, is that a growing number of users are reporting some wonky behavior; it'll only be a matter of time before Apple responds. One MacRumors user suggests the problem could be stemming from units being improperly calibrated in the factory, which means this is entirely hardware-related.