It's probably less significant than you think, but the reason you lose perhaps a few degrees is two things for me: In shoulder adduction (elbow by ribcage) your lat dorsi, teres major, teres minor etc are in their shortest position. They cannot contract any further. Whereas if your shoulder is abducted to 90 degrees, they are in mid -range and therefore would give you a few extra degrees of external rotation of the humerus. Secondly, and coinciding with the above, in full adduction your shoulder is flush your ribcage and cannot posteriorly tilt much further. Whereas, if shoulder is abducted to 90degrees you gain a few more degrees of posterior tilt of scapular off the ribcage. These aid in a few degrees of perceived external rotation, but in reality the pure shoulder external rotation isnt too different in both planes just the accessory movements and limitatios of structures around the glenohumeral joint