Could just be the EPP. Sometimes, the higher speed could be listed first, and that's what it's using. Since it's 1600, Auto won't cut it. By default, it uses the 1333 strap, which would make it 1:1 with memory and CPU. So, you'll have to unlock it and leave your CPU speed at stock (if you want, which you shouldn't cause this is XS). After that, just clock the memory to 1600 which is the stock speed, and you should be good to go!