The ScanGauge does read exactly what the ECM is putting out. You can adjust it by going to the main menu. Press "MORE", then press "SETUP", then press the upper left button one time. It should now say "ADV SETTINGS". Press "EDIT", then press the upper left button 5 times. The screen should say "FUEL ADJ" on top and 0.0% on the bottom. Adjust the bottom percentage about 3%, run four tanks through, and check it again. You can adjust it as much as you want. Just go back to them win menu when you're done.
Since your ECM is just a giant calculator, .4mpg accuracy is very good. Most are off 1.0 mpg or more. I tell drivers you can tune it in to about .3mpg. Very few engines are perfectly accurate all the time.