Is it comparable though? The way the tests run doesn't factor in aerodynamics and weight properly. A member of another forum started a thread about it because he was finding it extremely easy to beat the quoted figures in his Boxter. These tests don't factor in the slipperiness of let's say a Boxter to the comparably brick shaped Panda for instance.
Effectively they stick the car on a rolling road to run the tests so the car doesn't need to accelerate anything other than the actual drivetrain up to speed. So while yes it will give comparable measurements between a 1.3mj , a 1.2 and a 1.4 500 it won't factor in the extra bulk of the diesel engine which is why there's always a bigger gap between the quoted figures for diesels than there is for petrols.