I built a new rig recently - it's about 2 weeks old at this point. I noticed (when monitoring my temps through software) that the rated frequency for my 4770k has been consistently fluctuating between 4.1 GHz to 4.2 GHz.
I'm kind of confused as to why this is happening, because it's rated at 3.5 GHz out of the box and 3.9 GHz with turbo enabled. The kicker is that I haven't changed any BIOS settings nor have I done anything to actively attempt to overclock this CPU, yet it is running at a decently (from what I've seen) stable clock speed. I believe the base frequency is around 100 MHz and the multipliers on each core were anywhere from between 41 to 43.
My setup is as follows:
Mobo: Asus Z87-Plus
CPU: Intel 4770K
RAM: 16GB Corsair Low Profile (1600 Mhz)
GPU: AMD FirePro V4900
PSU: Corsair TX-750M
Cooling: Corsair H55 (2x Corsair SP120 High Performance attached), 2x Corsair AF140 Quiet Edition, 1x Stock Corsair 120mm (the one that came with the H55) and 2x Stock Fractal Design 140mm case fans
The only reason why I assume the CPU to be stable at this clock is because I'm getting anywhere between 27-28 to 33C for idle temps (my room is the hottest in the house and we don't have AC. It all depends on how the temperature/weather is outside) and anywhere between 55-65C at 100% load for when I'm pushing some renderings. I tried running Prime95 with this rig for about 2 hours and I didn't get BSOD - I think the CPU may have peaked at 70C with this test.
Would anyone know why I'm getting the performance of an OC'ed system even though I haven't OC'ed it? (not that I'm complaining)
If you would have bought it from somewhere, it would have made sense that they overclocked it.
But if you built the system there is no way for the CPU to gain higher clock speeds on its own.
There might be some over-smart kid in your family or one of the guests who did that just for fun.
Overall 4.2 GHz is very stable speed for 4770K, in fact all Haswells since the highest stable speed in almost 90% of the Haswell CPUs is 4.4 GHz with 4.2 - 4.3 being ideal.
However I highly recommend you to set the multiplier to 16 - 43 because that would let the CPU throttle down when high power is not needed and thus would be good on electricity bills and overall system health.