Shaped mirror glass changes the order in which a mirror has to be built, and getting that order wrong is how shaped mirrors go wrong.
A rectangular plate can be ordered to a measurement and the frame built around it, because a rectangle is fully described by two numbers. A shaped plate is not. This arch has a curve at the head, a change of direction at each shoulder and a return down each side, and no set of dimensions communicates that to a glass cutter accurately enough for a rebate to close on it.
So our master Indian artisans build the frame first. The carving is completed, the rebate is cut all the way round the finished opening, and only then is a template taken directly off that rebate - the actual curve, not a drawing of it. The glass is cut to the template.
Cutting the glass first and then carving a frame to match it is the wrong way round, because a carver can be asked to follow a line by a fraction and glass cannot be persuaded at all. Build the timber, then serve it. That is the whole rule.