One person says that the potential energy of a particular book kept in an almirah is 20 J and the other says it is 30 J. Is one of them necessarily wrong?
No, there’s no necessity that on should be wrong in their claim. The potential energy of the almirah is dependent on the height at which the book is kept. Therefore, the observers might be present at different reference point relative to the book hence measuring different potential energy of the object.