[JaimeThompson]

The phone may be using GPS for headings instead of an actual compass causing it to act that way.

[fingerlocks]

Why would it do that? Digital magnetometers are cheap, low power, and ubiquitous.

[mkl]

And also, IME, completely unreliable. The compass in my car's sat nav never points north, and my phone's compass constantly needs recalibrating.

[JaimeThompson]

Some inexpensive phones don't include it due to cost reasons, it could also be bad software or a defective magnetometer in the unit.