It is called vdroop. It is a feature in motherboards that prevents accidental over-volting of the CPU.
vdroop happens most when the cpu is under load (ie, stress testing like prime95). For example, i have my vcore set to 1.42v, but when under full load it dips as low as 1.37v.
When overclocking, the voltage in CPU-Z would be the most correct. I would suggest downloading prime95 for stress testing as well as HWMonitor to monitor your temperatures.