One Rail, No Corners
A rectangular sofa is straightforward to build because it is made of straight parts. Cut a back rail, cut two arm rails, join them at right angles, and any imperfection at the joint disappears under the upholstery. A tub sofa removes that safety net. The top rail is a single continuous curve in show wood, visible along its whole length, so it cannot be assembled from short segments with the joins buried, and it has to hold a fair line the entire way round. The eye is remarkably good at finding a flat spot in a long curve, even when it cannot say what is wrong.
What you get for that effort is a different experience of sitting. The enclosure comes round past the shoulders on both sides, which makes a large sofa feel sheltering rather than exposed, and the curve turns the outer occupants slightly inward so three people can talk without leaning forward. It is also why the carving on this piece is deliberately sparse. Heavy ornament along that rail would chop the curve into episodes and destroy the continuity the whole form depends on, so the carving is confined to the centre crest, the arm brackets and the apron. Every part is shaped, carved, finished and upholstered by hand by master Indian artisans.