The giant regulator is actually the governors of the steam turbine generators at the power plant maintaining a constant rotor speed. As load increases, generator rotor starts slowing down due to increasing flux fields, which causes governor to admit more steam to generator turbine to maintain rotor rpms. In a sense you are correct because as more power is needed, more steam is used, steam boiler pressure starts to drop, and a fuel pressure regulator opens wider to maintain boiler pressure.Right, of course. I suppose since the load is in constant flux (as thousands of people turn things on and off), so is the voltage. I'm now picturing a giant regulator of sorts, somewhere in the mix!
So when manufacturers provide spec's for their products, they just pick one of those voltages and they're common enough that everyone "knows what it means," and it is implied that the product will work without issue within some variable range, I guess. Thanks James.