what your are seeing is the Speed step technology. When the full power of the CPU is not required it down clocks itself "as such", so it does not use as much power. So yes it is normal, i believe you can disable it in BIOS.
if you want to read more, i found this after a quick google, i haven't read through it but i'm sure it will explain it better than me: http://en.wikipedia.org/wiki/SpeedStep
Hope this helps