the range calibrates itself based on the mileage you have just done on a tank.
e.g. If I fill up and get a range of 280 miles, I then use the fuel getting the cruising range down to 0 and fill up again I get a new range of the mileage I have just covered, say 275 miles. If I then use up the fuel again, let the crusing range get down to 0 (now covered around 270 miles) and then drive for another 40 miles when I fill up the new range is 310 miles.
Basically the range (and the mpg figures) are inaccurate.