Lets refraze . A Voltage regulator gives a constant current. But the resistance of the CPU goes UP when is in full load, and because of it, the voltage must go UP too (U=I*R), but ... it's not rising fast enough when oveclocking. Why ? thats a lot to write of
Because of that, the voltage of the CPU must be set higher when overclocking, and then under full load of the cpu Voltage drops to compensate the current needed (thats why is called Vdrop ). Very simple explained but is more complex involving also the PSU, but not getting in details about that.
One solution is to rise the current when oveclocking, or to make a voltage-current regulator (a little bit harder to produce, and more expensive than a simple Voltage one).
So untill then, we are stuck with voltage regulators and that what is happening.