It looks like the EA-1600 shares the same case. The EA-1600 is an wireless AC router, vs the 1200 which is a wireless N router, so one would expect the EA-1600 would use a larger PCB and need more space. Note the extra clips and standoffs in the teardown of the 1200. Saves cash on the design of a new case, design of injection molding tooling, making of the molds, and logistics/supply chain of yet another case style.