It's probably because you have TurboBoost enabled. This can augment the multiplier by 1, and can modify the bclk. All in all, this means more performance, more voltage, and more heat. If this poses a problem, you can disable it in the BIOS. It could also be that you have some power-saving options activated in said BIOS. This lowers the vcore/multiplier/bclk when the CPU is not used very much, leadin to less heat and less power consumption. You can also disable these options in the BIOS.
For more info : http://www.intel.com/technology/turboboost/
Hope this helps !