The standard speed for DDR3 (from a specification perspective) is 1333. Anything faster than that is technically overclocked.
Regardless, the performance difference between 1333 and 1600 for most users is negligible.
For kneeman, some motherboards will automatically down-clock memory, for optimal stability, when all memory slots are populated. It is a matter of power management and stability.
In non-OEM systems, users can manually override these settings. In an OEM system, you may have to live with the 1333 settings.