What does compass calibration do and why is it needed more than once?

The"compass" in Android seems to be a projection of the 3D magnetic field measured by a magnetometer : Having read https://electronics.stackexchange.com/a/22271/282833 I still have three questions: 1. Assuming the compass has been calibrated while quality testing before shipping, why is it needed to calibrate the compass again? Or in other words: Assuming that the magnetic sensor is placed the same with high precision on each device of the same series, why can't a "golden sample" be calibrated and the data set to all of the similar devices? 2. Assuming you calibrated the compass and you use it in more or less the same area (meaning: I know that the compass may need re-calibration when moving a significant distance, like a few hundred km), but same area means less than 50km), what are the reasons you must re-calibrate the sensor after some time? 3. Is the compass calibration global to all apps, or do individual apps require separate compass calibration? It seems some apps suggest re-calibrating the compass frequently, while others don't. Background for the question is that even after calibrating the compass, it started to "go mad", meaning north was almost 180° off.