Every flute in this back is stuffed separately, and each one is filled a little more than the last towards the centre.
The reason is the shape of the back. It curves gently forward at its ends, and a channel that is filled with exactly the same amount of material along its whole length will look thinner where the back curves away from you and fatter where it faces you square on. The eye compares each flute with its neighbour, so any inconsistency reads immediately as a mistake.
So our upholsterers weigh the filling for each channel and increase it slightly through the run, compensating for the angle at which each will be seen. The result is that from a seated position across the room, every flute appears identical - which is exactly what nobody notices, and that is the point.
The buttons at the base of each channel are then pulled to a marked depth rather than to feel, so the row of buttons sits on a true horizontal line even though each is anchored into a slightly different thickness of filling.