Sandy can only really handle 1.4v 24/7 because of electromigration issues. I'm no physicist, but from what I understand, all the electrons that we slam through our CPUs begin to actually move physical matter i.e. the chip. The more energy you give the electrons, i.e. the more volts, the faster this process becomes. The movement of this matter destroys circuits, and can thus render the microarchitecture of CPUs worthless.
The added material in the wrong places create more resistance, and this, I would assume, is why you're finding yourself needing more volts to achieve the same clocks over time.