One of the other things to remember with the Vivi Nova is that the wire connections are not solid like a soldered connection. The wires are held to the metal connection by rubber pressing them together. This can affect the resistance because a tighter connection gives less resistance and if it loosens, the resistance will increase. As stated before in this thread, the resistance will also increase and decrease as it heats and cools.
This is why variable wattage is such a Great advantage. You set the wattage that tastes best to you and VW will adjust the voltage to keep the same watts as the resistance goes up and down with the temperature. The taste remains the same without you adjusting anything as it goes along. It is the way of the future, for sure, but it is already available at reasonable prices ($40 - $100).