Actually if I am not wrong, the manufacturers quote the FC under quite perfect conditions, which means not much load, long distance driving, etc etc. That is why in reality, u seldom get the FC that the manual says u will get for your car.
As for the exact conditions, I am sorry, I don't know also..

But I guess you can compare between manufacturers, as they should have used roughly the same (near perfect) conditions.
But comparing between drivers is nearly impossible as like u mentioned, the distance, aircon, load etc all make a difference.