DDR RAM has a data rate which is twice the actual clock speed of the ram. So the ram runs at a clock speed of 333.33 MHz, which translates to a data rate of 666.66 MHz (2 x 333.33), which is rounded up to 667 MHz because of controversy with the number 666.
In short, your RAM is running at the speed it should be. The motherboard is talking about the data rate of the RAM, but CPU-Z always shows the clock speed.