I asked my dealer about this. It turns out there's a rubber bladder in the fuel tank. If it's cold when you fill up, the rubber doesn't stretch as much, so less gas goes in (and the mileage goes up). When it's warmer, the opposite is true (more gas can go in, the mileage goes down). The computer's calculation is not affected by the bladder (I think the mechanic said it uses numbers from the fuel injection?) I saw another post on this board that mentioned someone doing some calculations regarding this.