Could be that it's his first fill up. On the first fill up, you are NOT supposed to adjust the gallons, only on SECOND and SUBSEQUENT fill ups. But, on that note, I don't think it would keep you from trying to adjust it, which is why it tells you specifically not to...
And also, if you go into the TRIP features, if you like any of those displays, you can program them like an X-Gauge, and then display them on the GAUGES. For instance, one I like to have is Distance to Tank Empty. I programmed this as an X-Gauge, and named it DTE. You can also get MPG Average for the Tank, if you wanted.
Also, do not forget to calibrate your speed in the options. I think with my +1 size tires, I was 3% off in speed. I just used my phone and displayed MPH on the Waze app, and got on the highway, and set the cruise control. Monitored when the MPH switched between say, 64/65/66, and tried to get ScanGauge
to change at the same points. If you have bigger tires, this will make a difference in your readings, and of course, you may notice a slight difference in your odometer calculated MPG, and the SGII calculated MPG for a tank, in that case.