On a swept frame the corners are the honest test, and the answer is decided before any carving starts.
The hollow that runs along each rail cannot simply continue into the corner - two hollows meeting at a mitre produce a soft, muddled junction that never quite resolves. So our artisans stop the sweep short at each end and leave a flat pad on the corner block. The hollow is then worked from the centre of each rail outward toward that pad, and finished with a scraper run in one continuous pass so there are no facets in the curve.
Those flat pads are what the corner cartouches are bedded onto. The ornament is not stuck over a curve and hoped for; it has a true, flat seat cut for it, which is why the leaf clusters sit tight to the frame with no gap under their edges even after years on a wall.
It is a decision nobody sees. It is also the reason the frame still looks right in a decade.