Not too familiar with Intels, but do you have something that is like the Cool n Quiet technology with AMDs that throttles the CPU down when it isn't doing much work? I'm wondering if that's what you are seeing here.
I know with the AMD, if I enable Cool n Quiet, I'll see my speed go from 2.8 ghz down to 803mhz, especially when the system isn't doing anything.
Have you tried running all three monitoring programs at once to see if they match at any time? I think what you are seeing is the system increasing or deducing the amount of power the CPU needs to run at dependent upon the work it's doing at that time.