The stud line on this chair has to turn corners, and a corner is where a stud line either looks considered or looks like it ran out of room.
At an outside corner - the front of an arm, say - the studs on each face converge, and if the spacing is simply continued round, two studs end up almost touching at the turn while the rest of the run stays even. At an inside corner the opposite happens and a gap opens.
So our upholsterers set the corner studs first. One stud is placed exactly on the turn of every corner, and the runs between corners are then divided into equal spacings that arrive at each corner stud precisely. The pitch therefore varies very slightly from run to run, and every corner is resolved by a stud sitting on it rather than beside it.
It is the same principle as setting out floor tiles from the centre of a room rather than starting at one wall - fix the points that must be right, then distribute the difference where nobody will measure it.