You are just seeing where the system manages itself (even with CnQ disabled, I recommend you enable it, BTW). If you were stressing the system, you would likely see the memory speed increase. All modern systems increase/decrease component speeds (CPU, GPU, memory) as needed. It is an effect of power management, etc.
The standard for DDR3 is 1333, so setting to AUTO means running at 1333. To run at 1600 requires use of XMP (for mobos/memory that support) to get the mobo to config, or you have to do it manually.
The performance difference between 1333 and 1600 is not perceptible by the user. As a matter of advice, I never recommend overclocking memory (and to run at the rated 1600, you are technically overclocking) because the gains don't outweigh the negatives that can occur (heat, power, stability, lifespan, etc). Overclocking CPUs and GPUs, I am okay with. Your call.