I think there is an agreement here that Time Under Tension (TUT) is of great importance when working delts, namely that they seem to require a given amount of TUT, larger than most muscle groups.
I agree, although I have not been doing the Arnold Presses in my current routine, have used them, as well as other TUT-increasing techniques, with good success. Interestingly, the idea for this came from growing my forearms, then my calves with success, using TUT increases. My conclusion is that the denser muscle groups - Calves, Forearms and yes, I do believe that delts, while not as dense as the calves, are denser than most other muscles and benefit from more TUT.
As to WHY that would be the case, well I wouldn't know.
Does that make sense to you, Bobo?