I have seen similar results with my 04 as Tekrep reported using the same procedure. But if I run say half a tank purely on the highway and run the rest of the tank in the city, it seems as though the actual mileage calculated by hand is about 1 mpg off from the computer.

Here's some food for thought. Assume that you travel 480 miles and get 15 mpg. That would translate to 32 gallons of fuel used. Now if you squeeze an extra gallon of fuel into the tank either to round up to the next round dollar figure or to just top off the tank, your calcuated mileage drops to 14.5 MPG. My point is that very small changes in the amount of fuel you pump translate into larger changes in the calculated fuel mileage.