It's important to understand that your CPU has a base clock speed, which is guarantee the manufacturer gives if the minimum cooling requirements are met. Anything above this speed, the so-called turbo speed, is a freebie that's not guaranteed, so fluctuations between the base and highest advertised turbo speed are expected. Also, the more cores you load, the lower the turbo speed the CPU will go to because there's still a maximum power and thermal limit and the CPU cannot run all cores at the maximum turbo boost speed because of this.
There's also the opposite end of the spectrum where the CPU will lower its clocks if it's not doing much to conserve power.