I think what the OP may be referring to is the fact that he was used to the behaviour on X58 series boards but with the later X79 hero and 4790K the behaviour of cpu voltage is not quite the same.
Sherriff - You need to be mindful that one major difference over the X58 mobos & CPUs is that the X79/4790K combo has had part of the voltage reg circuitry moved to the CPU itself [FIVR - Full Integrated Voltage Regulator]. The CPU will actually control the core voltage to some extent even though its set to fully manual on your board - this is normal.
Use HWINFO for measuring Vcore as it seems the most accurate.
The cpu can vary the volts slightly which it does when under load. Your best bet is to pre-empt the amount of difference in voltages when idling Vs under heavy load [Prime95 is a good load...!]. Also - Manually set your cpu volts rather than relying on Adaptive or Offset modes when overclocking. You can switch to Adaptive/Offset once you know exactly what range of voltages your CPU is stable at your chosen overclock frequency by factoring in the difference between idling and full load. Its a lot more complex to setup Adaptive/Offset modes when overclocking and in some cases it will introduce instability - especially if not set correctly so a lot of overclockers do not bother using those modes and ultimately stick to manual core voltage control.
Here is a snapshot of my
manually set Vcore at idle and then under load [Prime95 v28.5 - Small FFT Torture test].
Mobo is Hero VII with a 4790K - ie same setup as yours.
It's pretty warm today [approx 28C here in my room atm] so excuse the slightly highish cpu temps...:)
Note that VID = Voltage set in Bios [1.31v] and Vcore is the actual voltage supplied to the CPU. So only refer to the Vcore voltages near the bottom of each screenshot and not the VID voltages.
@ IDLE - Note VID and Vcore volts are pretty much the same.
@ Prime95 Load [v28.5 Small FFT] - NB: Only run this test for extended periods if you are kitted out with a very good cooling setup [Pref. a high performance water cooled solution] as the CPU can easily reach in excess of 100C in seconds. My 4790K has been delidded and runs in a custom water cooled loop. You can see my temps still ramp up to in excess of 85C pretty much as soon as I execute P95. Although its quite warm in my room atm - around 28C due to it being summer here Downunder - Should go and switch on my air conditioning...!
Note that the core voltages have jumped by approx 20 to 30mV which is normal when under heavy load.