Is your (lack of) scapular stability cheating your thoracic mobility?

This morning, as part of this whole scapular stability/thoracic mobility project I’m working on, I noticed something interesting: I’ve been cheating the quadruped rotation exercise. The picture on the left is of me at the top end of my left side rotation range of motion, and the picture on the right is of me at the top end of my left side rotation range of motion after setting my scapulae.