If its anything like Intel CPU's they have power save features. My i5-2500k runs at about 1.6GHz if i remember correctly when under no load, then if an application needs more resources it will bump itself back to 4.6GHz (voltage changes accordingly).
If your PC doesn't need to be using all cores maxed out, it will use a much lower CPU Frequency and Voltage. This saves you electricity and helps with your CPU's life-span since its not running balls to the wall 24/7. Try running a program, something like Prime95 or another demanding application and you should see your CPU using its higher potential.
This feature can be disabled within the BIOS, however it makes sense to use it.