The more accurate name for the list is "Manufacturer Adjusted Fuel Economy", and it comes as part of the Light Duty Fuel Economy Trends Report that the US Enviromental Protection Agency releases every year. Manufacturers are assessed by the average fuel economy of all the vehicles that they currently offer over a one year period. If that sounds a little dodgy, don't worry - the fuel economy is independently tested and verified, but the adjustment comes in the form of various equations and methods that the EPA specifies to account for various technologies.
You see, testing the fuel economy of cars based on current standards and methods may not fully exploit the technologies that a car comes with. A plug-in hybrid vehicle may not be tested with a full charge of battery, a car with a stop-start system may not have it active during the test period, and so on and so forth. As a result, the EPA does allow a little bit of leeway with what they deem is a reasonable improvement in fuel economy for these cars.
In any case, Mazda is in top position for this list, and this would be their fifth year in a row at the top. Mazda's vehicles have an average of 29.6 miles per gallon (7.9 litres per 100 kilometres), which is a 0.4 mile per gallon (0.16 litre per 100 kilometre) improvement from the previous year. This figure is only going to improve in the coming years with technologies like SKYACTIV-X and their commitment to electrification going full swing, although there's no doubt that other manufacturers will be catching up quickly.