2009 4,1–5,1 Mac Pro Upgrade Only Shows 1066 on RAM. NVRAM Reset Didn't Work
I can't find out what actually decides it, however I just noticed this note on the Apple KB - Mac Pro (Mid 2012 and earlier): How to remove or install memory


  Note:   Although the memory speed of the UDIMMS is 1333 MHz, they will
  operate at 1066 MHz or 1333 MHz depending on the processor in the
  computer.


Typically, MacPro5,1 models with one of the Nehalem processors only support a memory speed of 1066 MHz, while those with one of the Westmere processors support a memory speed of 1333 MHz. In your case, the X5675 processors are one of the Westmere variety, so I can't explain the discrepancy in speed.

I have a pair of X5690 in mine &amp; RAM runs at 1333MHz.

It's probably worth making sure you have the pair in the correct places, just in case...



BTW, the Pros run fastest in triple channel - so if you can afford one more RAM stick...