In some of my photos of the building process, it appears that the two springing points of Crown Bridge are the same height. In this photo of the side of the bridge having all the copes now completed, you can see that the right side of the arch starts much higher than the left.
The voussoirs stick up in a jagged pattern above the line of the arch. These protruding rows are thoroughly pinned with thin shims to tighten the whole network of radiating stones in the vault.
A covering of medium sized stones are then laid in place above the mesh of voussoirs and shims.
Then a 3 inch layer of 3/4 clear stone is put over the entire walking area between the copes and smoothed to create the base for the path.