Quite a lot of lower-end motherboards don't support RAM speeds above 1333mhz without overclocking, check your motherboard manual for more info.
Not that it matters anyway, unless you're doing some extreme overclocking for the sake of benchmarks you aren't going to see any practical difference between 1600 and 1333 mhz (in fact some overclockers have found that they get a performance boost by down-clocking from 1600mhz and instead tightening the timings).
You probably have a CPU or chipset than only supports RAM up to 1333MHz by default. 665*2 = 1330, so that would be about right. You will more than likely need to raise the frequency of the RAM in the BIOS to 1600MHz. This will probably make the system unstable, so I would advise doing this unless you really know what you are doing.
For example, Sandy Bridge chips only support up to 1333MHz RAM. So, no matter how fast your RAM is, it will always be underclocked to 1333Mhz by default.
Actually you can. I have an i7 2700k (Not overkill it was free upgrade from i5-2500k) and I just got the ram to 1600MHz in the BIOS. I see a slight performance increase like 4FPS. Sandy Bridge processors CAN support up to 2133MHz.
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