I'd be very surprised if any of the mileage ratings are accurate. They're all done in laboratory settings simulating very specific things.
Just because your car is rated at xx highway, doesn't mean that's what you'll be getting. It's more of an aid for comparison than accurate ratings.
That being said, it would be nice if the rating had an error bar attached to it. Something like 5.5 l/100 km +/- 0.5 would be helpful