If you are trying to compare it to your GPS, it all depends how accurate your GPS is. Currently, the government states that most modern GPS's are accurate down to about 5 Meters (~.003 miles), but then buildings, terrain, other satellites, etc. (not counting software inconsistencies) can cause a deviation from that accuracy as well (pretty much where the .4 difference can come from).
I wouldn't bank on a GPS system to calculate actual miles. A car's computer that can compute (if programmed correctly) the wheel radius, vehicle speed, driveline ratios, etc. would be more accurate. Also, i noticed there was no mention of change of wheel or tire size, but that could open up another can of worms on this thread. If you feel something is off, a dealer can reprogram the speedo if needed.