They've stated now that those figures are only lab test ones, and that they shouldn't be taken at face value but instead used to compare between cars.
For example, you have a 320d that states 60mpg and an S1 that states 50mpg, neither will get anywhere near that but there will be 10mpg between them. So in the real world the 320d gets 40mpg, the S1 gets 30mpg.