The four carved bosses on this frame are not where the arithmetic would put them.
Measure the frame overall, including the crest above and the cartouche below, find the halfway point, and put the side bosses there - and they will look low. That is because the eye does not measure the frame. It measures the bright rectangle of glass, and it reads the crest as something sitting above the piece rather than as part of its height.
So our artisans set the side bosses to the midpoint of the plate opening, not of the frame, and then check it by eye on the assembled piece before the carving is finally cut in. On a tall mirror with a big crest the difference between the two positions can be considerable, and it is the difference between a frame that looks balanced and one that feels subtly wrong for reasons nobody articulates.
The same logic decides how far the corner scrolls break outward and how high the crest can go before it stops reading as a crown and starts reading as a second object. All of it is set out afresh for every size we are asked to make, because none of these relationships survive being scaled up or down.