The Shoulder

make sure that the technical execution of the test is perfect and compare several times with the other side.

an asymmetry in the shouldergirdle can give the impression that the lateral rotation is being carried out less thoroughly. In that case, a good point of reference is the position of the forearm in relation to the spina scapulae. When a light inclination of the scapula is the cause of a diminished exorotation, then, a relatively enlarged medial rotation will make sure that the combined exo-endo movement has the same range on both sides.
when further in doubt: simultaneous passive bilateral lateral rotation in supine lying position. This gives a better relaxation and a better fixation of the shouldergirdle. By carrying out the movement bilaterally, a limitation can rather easily be discovered.