I think official MPG figures are always going to be off, because you will never be to replicate the same drive twice. Your MPG will change based on many factors, temperature, cargo, how fat you are, avg. speed etc etc.
The reason it is so good on a motorway is because your not constantly accelerating to bring the car back up to speed, ie not using as much fuel! If you were to do 90 on the motorway you would be using more fuel to stay at a higher speed and your MPG would be lower..
This is why threads like this are so good because you can read how people drive vs what MPG figure they get, then compare (or sometimes decide to buy a car!).