[Phantom]

As said above that it depends on several things one that the miles is correct another is how often it updates the totals and what is the max MPG it will use is it 99.9mpg or 999.9. Also the differences is based on how the fuel injectors are flowing compared to what is expected, if one is flowing more or less that the others it can calculate it wrong also if the PCM table that maps the flow of fuel expects more or less than what the average of the injectors is it will be off at that point.

Remember that the fuel injectors flow in not linear so some places the predicted flow to actual flow may be different at different loads and changes in throttle position.