The back of this sofa comes to a point at the top, and that point decides where every button below it goes.
On a rectangular back the buttoning is set out from the seat upward - it is the simplest datum and nothing above cares. Here it cannot be. If the grid starts at the bottom, whatever row happens to arrive near the apex will land at an arbitrary place, and you will end up with either a button sitting slightly off the centreline at the point, or a gap where the eye expects one. Either reads instantly as wrong, because the apex is exactly where a viewer looks first.
So the setting out is inverted. The topmost button is placed first, dead on the vertical centre line, a fixed distance below the point. The grid is then built downward and outward from it, and any leftover fraction is absorbed at the bottom edge - which is hidden behind the seat cushion where nothing shows.
It means the whole diamond pattern hangs from the apex rather than standing on the seat. That is why the point of this back looks resolved instead of accidental, and why the centre column of buttons runs true all the way up into it.