Deep foliage carving is built backwards. The first thing established is not a leaf but the ground behind all of them.
A surround like this has three or four layers of acanthus overlapping one another, and each leaf has to sit convincingly in front of or behind its neighbours. If the carver starts with the leaves that will end up in front - the obvious place to begin, because they are the ones you see - there is no way to know how much depth is left for everything behind them. By the third layer the timber has run out, and the back leaves get flattened into the ground.
So the deepest ground is cut first, right down to its final level, all the way around the frame. That fixes the total depth available. Then the layers are worked forward towards the surface, one plane at a time, with each layer completed all round the frame before the next is begun.
It means the frame looks unfinished for most of the time it is being carved - a set of vague masses at different depths, with no detail on any of them - and only becomes foliage in the last stage.
It also means the leaves at the back are as fully modelled as the ones in front, which is what gives the shadows their depth.