The iPhone 5s includes a gyroscope, accelerometer and compass, meaning it can variously be used as a spirit level, direction finder and fitness tracker. However, testing suggests that the accuracy of its measurements is poor -- so don't use the spirit level app if you're trying to fix a shelf.
Following widespread online reports of inaccurate data from the iPhone 5s, Gizmodo ran its own tests, comparing the performance of the device to both real-world measuring tools and to the iPhone 5, which includes much of the same hardware. In every case, the iPhone 5s produced dubious and inaccurate data. As well as messing up real-world applications, that's potentially very bad news for gamers, since many iOS games rely on using the gyroscope and accelerometer to read user input.
The most likely explanation would appear to be improper calibration during the manufacturing process, which means that the issue can't necessarily be fixed purely through an iOS update. Hit the post for full details of the testing.