Looks to me as if the center panel with the zipper is a separate piece of fabric from the side pieces. This means the pieces were cut carefully with attention to keeping the pattern of the fabric intact. The center piece would have the zipper inserted while the piece is still flat and on its own. The fabric would be slit, then a piece of facing sewn with a narrow seam (say, 1/4") to each side of the slit. The zipper would be sewn in, leaving a seam allowance on each side. The side pieces would be seamed to the center piece, and the joinings would be covered by the handles.
My best guess, anyway.......