I always thought that relative info gives us information about the object relative to the object's local coordinate system. And by this logic, if both objects have Y rotation 0, then relative rotation should give 0 in Y.
[![enter image description here][1]][1]
[![enter image description here][2]][2]

Then how to understand this?

[![enter image description here][3]][3]

How is relative info actually converted from original info, if not by subtraction?


  [1]: https://i.sstatic.net/jpPDq.png
  [2]: https://i.sstatic.net/Ei3kt.png
  [3]: https://i.sstatic.net/wRnSb.png