I can only guess to a few things. I haven't read what that board supports, however:
Either:
The FSB is running at 266 and your ram option in the bios is based on your FSB - in this case you're running at 1066.
Or:
That board doesn't support that ram.
Or:
You haven't set your ram settings manually to take advantage of the ram. The board possibly supports 1333 ram by default - you'll need to overclock a tad to use up and reach 1375. Good luck.