The reason for the difference in height is to make them easier to build: It's not obvious that there's a 1mm discrepancy between the component heights if the drawer side is 12mm lower than the front. It also makes them easier to fit into the drawer hole (I'm sure there's a better word in English than "hole")Why do furniture makers insist on making drawers with low backs and sides or just a rod for the sides ?
Can you imagine the fuss it would cause if the Hatton Garden Security Vault used such drawers, with contents spilling over into the drawers below ?
The rod for a side is a new one to me though. I'm guessing it's one of the tricks on mass produced furniture.