The answer is that the potential range is measured against a standard driving route and real life is different, so the figures are no more than a comparison between models. Magazine tests have shown that you are unlikely to get these figures if you buy one though. Using the heater and lights in Winter will seriously reduce the range so Winter use of many will be seriously worse than the figures used by testers.
Having said that our neighbour has just bought one as a second car, which seems to be the best option for these cars in most cases and he can actually run it for nothing. Basically the car chargers are free so he drives to the one nearest our local Tesco plugs it in and does the shopping. When he comes back he has a near full battery, although I think both of us realise this will not last for ever. There are plenty of other sites he can use so in the short tern at least he can keep the costs right down.
Our council has bought an electric dustbin wagon and says that the diesel equivalent cost £100 a day in fuel but only £15 for the electricity so the savings are there for those who can work the system. The catch though seems to be if you have to recharge on the road, as the costs there tend to be near the cost of refilling with petrol so long distance touring though possible is not as cheap.