iPhone - understanding iPhone rotation

Question

I am banging my head on the wall trying to understand this. See the next picture.

Suppose I have an iPhone resting on a table. At this time the rotation readings thr

Answer 1

Euler angles are not that precise for decribing arbitrary rotation like quaternions or rotation matrices. There is often more than one set of angles to decribe a rotation and thus it's not unique.

[Edit:] As stated by Harinder core motion converts the angles into canonical representation i.e. there are some constraints - I am not sure but think the pitch angle is limited to [-90;90].

Have a look at Gimbal Lock or some similar articles about the phenomenon.