I think there are 2 reasons for this:1 The entire tank contents are not available for use. Most fuel tanks (cars, aircraft, ships, fuel oil etc) cannot drain out entirely (maybe to avoid airlocks in the fuel lines) and so there is always some left in the tank that is unusable.2 There is a further usable reserve in the tank once the computer range hits zero. On other Audis I have owned, I have driven the car up to 5 miles after the computer recorded a zero range (I know it's stupid! It would be interesting just to see how far the car would travel after a zero range was shown, but that would entail draining the fuel lines, which I think can cause damage to the catalytic converter, or something like that, as well as introducing airlocks into the fuel lines. I suspect that there is about a 10 mile reserve, or about 1 litre.Of course, there is an argument that the specified fuel capacity should be the usable volume, rather than the nominal volume, but that will never happen (think computer disc drives where a nominal 100Gb drive actually only has, say 95Gb usable). Also, the nominal capacity is a known quantity (i.e the volume of the tank), whereas the volume of fuel can expand/vary with temperature and pressure. Finally, when filling the tank, do you stop filling when the pump handle trips, or do you trickle feed to the point where you can see fuel in the filler neck?