Carving a flat border is harder than carving a shaped one, and the difficulty is invisible: it is the depth of the ground.
On a moulded frame the profile itself gives the carver a reference. On a flat face there is nothing to work to, and a ground that wanders even slightly - a fraction deeper along the bottom rail than the top - will make one side of the mirror look heavier than the other under a lamp. Nobody could tell you why. Everybody would feel it.
So our artisans scribe a depth line along the edge of the frame stock before any pattern is drawn on it, and cut the ground down to that line first, across all four sides, checking with a small gauge as they go. Only when the ground is dead level does the flower and scroll work begin on top of it.
It means the pattern is built up from a true floor rather than dug down to an approximate one. Which is why the border on a piece like this reads evenly all the way round, and why the beads at the two edges stay the same brightness from corner to corner.