It is in fact largely nonsense though, I always find my mpg is higher than reported on those sites, and owners clubs etc.. It very much depends on what kind of driving you do and how you drive.
So many times I've got into peoples cars and found that they have this bizarre driving ADHD when they get behind the wheel. Constantly wanting to accelerate up to the back of someone, brake, change down and repeat. I find it very odd, you only need a little bit of patience, leave a reasonable gap, read the road ahead (yes that means 10 cars in front, not just 1) and you can maintain a constant speed and drive calmly without being agitated. This has the benefit that you are more efficient with the use of your fuel, tyres, brakes, and ultimately you don't become one of the people that exacerbates the congestion that you are so keen to get ahead of.