The ruching around the seat edge of this chair is a measured thing, not a gathered accident.
Ruche is made by taking a length of cloth considerably longer than the edge it will cover and pleating it down to fit. The ratio between those two lengths is what decides the look: too little and the gathers open into gaps when someone sits down; too much and the edge becomes a thick roll that swallows the carved apron below it.
Our upholsterers work to a fixed ratio for this design and mark the pleat positions on both the cloth and the frame before any stitching, so the gathers arrive evenly around the whole seat rather than bunching at the corners - which is where fullness always wants to collect.
The pleats are also set to fall vertically, in line with the legs beneath. A ruche that runs at a slight angle is the commonest fault in this kind of work and it makes an otherwise good chair look tired from the day it is finished.