I wondered what people thought about this.. here's my .02,
Being a 13yr ase certified master tech and speed shop owner turned scca WC GT tech, I know a fair amount about what goes into calculating fuel mileage and what parameters of the fuel maps meter fuel when. The cube is very accurate from my view point on its mileage. However, there are spots in the "instant" calculations that I find holes in. The biggest is cruising idle compared to cruising full decel. A vehicle when at speed with zero throttle input actually will stop injecting fuel into the cylinders. A vehicle at speed when at idle will be injecting fuel, significantly. The cube doesn't account for this based on its instant reading. The computers basic fuel mapping is based mostly on air flow in cubic feet per min over inHg of vacuum, plotted over rpm. My guess is the cube only uses throttle position over vacuum. Other cars calculate it differently and could possibly be more accurate. Whether it REALLY matters or not, I couldnt tell you. If nissan engineers say that its more accurate than manual calculation, then so be it. My interpretation probably isnt significant enough.
On the fill up note, the only way to accurately gauge the fuel mileage is to use the same pump, at the same station and fill the car till it vapor locks and kicks the pump off. The design of the fuel filler tube and check valve will cause the pump to stop at almost the exact same time, every time. So, fill up, reset mileage, drive your distance, return and fill up, subtract fuel used divided over miles driven and boom, you got it.