Scangauge II Question

How does the SGII read mileage? Does it come from the truck's computer, or is it an estimate based on other information from the computer, such as burn rate?

I'm at about 250 miles for the tank and I notice there's a 7-mile discrepancy between the odometer and the gauge. I'm beginning to wonder if the odometer reads high (you know, so the truck will show more miles than it actually has in order to get to the end of warranty mileage faster).

Thanks.