Leaning is a function of the spine; it is not something you do with your shoulders. YOU DON'T LEAN WITH YOUR SHOULDERS, but with your spinal column.
What does your spinal column consist of? Your back and your neck. And since the movement of the head is totally integrated with the movement of the neck, the head itself completes the trial. Head, neck, and back comprise your spinal column.
Your shoulders just go along for the ride, and they are totally dependent on what your spinal column is doing. Here we have a woman leaning:
Notice that her shoulders are on an incline, but the axis of her head/neck is also on an incline. It is not vertical.