The 1600MHz is the max your motherboard will support in an overclocked state. Generally, the speed on the RAM, in your case 1600MHz, is the max speed the modules can operate.
To use the maximum speed of your RAM, you must overclock the modules. When you overclock your RAM, you'll probably have to play with the timings until you find a configuration your mobo likes.
for more info on OC RAM check out this thread.
Edit: fixed link