As the others have mentioned, quoted figures are done on a car 'treadmill' @ ideal temp, ideal wind (with fan) and ideal tyre PSI for max mpg. Plus also, no traffic! Plus all exercise buffs (like myself) know that running on a treadmill is way easier than running on the pavement/park, due to gradient, wind, surface etc. However what I can say is from experience, the previous generation cars quoted mpg was more accurate of real world driving, as manufacturers back then weren't as pressured to reduce CO2 + increase mpg figures.
Saying that....I would say it is very possible to achieve the figures (difficult, but it can be done) in the real word (if) when there is no traffic. Early on Sun AM or in the middle of the night. My 1.0 EcoTSI official figures are 53/64/72. When there is little or no traffic, 50+ mpg possible (once even did close to 60 in city), and combined of 64 too! See attached!
What you say is true though, it's easier to do the mpg in a diesel. My old Leon 2.0TDI DSG, 55/64/70 official mpg figures, i could easily get close to 60mpg without really trying on highways, and routinely got 64mpg on 40-50mph roads. On the petrol, i really needed to try hard to get close to official figures. I would say if drivers could hit the 'city' figures with their 'mixed' driving, this is already very good, as there is usually traffic, and traffic (esp crawling) kills mpg.
Out of curiosity
@Imperial...assuming yours is 1.4 EcoTSI (ACT) and comes as standard the 17 inch 40 profile Tagus rims? Also all my mpg figures are with Shell Vpower 99 RON. Like for like they gave me 3-4 more mpg all things equal compared to Shell/Esso 95 (or so the trip computer says)