A perfect VRD (Voltage Regulator Down) circuit, which is the circuity on the board that takes the 12V from the power supply and creates the 1.325V or whatever needed for the CPU, is possible but would cost more than the board. These VRD circuits have to be able to keep the voltage steady under changes in load current that are tremendous. For example your poor VRD circuit is happily sitting there thinking happy thoughts as you web surf and the CPU is doing basically nothing and drawing an amp or two. Nothing much is changing and if it does its not happening fast or very much. All parts of the VRD especially the filtering capacitors have some resistance associated with them even if just a little, lets just pull crap from out of the air and say a Cap has .1 ohm of resistance. If the VRD is pushing 1 Amp through that Cap on the way to the CPU, the Cap has a voltage drop of .1 volt and the VRD has circuitry that will compensate for that "lost" .1V , no problem.
You decide to fire up F.E.A,R. and hit "play" or whatever, almost instantly the CPU utilization goes to close to 100% and the current draw of the CPU goes from 1 Amp to 50 Amps or more. In a 1/1000th of a heartbeat the voltage drop across that Cap goes from .1V to 5.0V ! (Obviously this is a somewhat overblown example but the basic theory is true. ) The regulation circuit will compensate but it cannot do it perfectly or instantly because all this is happening in micro seconds and everything is changing. Due to some very complex electronic design issues (back to the Nothing can be Perfect law) the regulation circuit will "overshoot" the desired voltage before it can stabilize. This causes short term over-voltage spikes that can harm the CPU. (The same thing happens in reverse in going from full load to little or no load on the CPU but at worst the "undershoot" just causes the CPU to lock up and you reboot wondering what the hell caused your machine to crash, no harm done)
Intel is aware of this problem and publishes specifications and guidelines for standard VRD design. One of the specifications intended to protect the CPU is for a certain amount of Vdroop to be designed in. Whatever voltage the CPU is requesting the VRD gives it a little less to provide some safety margin for "over-shoot" during big changes in load current. Board designers can use these recommended values or design their own VRD but still must meet the spec to be Intel certified. Thats why it varies between boards, its all about the VRD design.
So if you leave Vcore on "auto" the VRD will typically deliver slightly less voltage than the CPU specifies and most boards act the same way if you manually set the Vcore.
This difference between what is "dialed in" and what the CPU actually gets is what the designer decided to "subtract" for spike protection and is called Vdroop.
(those "lost" voltages across components in the VRD on the way to the CPU are called Vdrop and can also cause the voltage at the CPU to not match the bios setting but that is a result of crappy voltage regulation and not as a result of a designed in safety margin. )
