I believe the reason it's even layered is because of screw size. With an odd layered puzzle the screw has to have a small enough diameter to go through the center piece. An odd layered puzzle would have had to be much larger in order to have strong enough screw to keep the puzzle stable. Since the "center piece" of an even layered puzzle is hidden, it could be large enough to make the puzzle stable without making the puzzle enormous. I could be wrong though.