The fuel consumption figure is a test to let you compare this car with other cars that had to go through the same test. It does not tell you what you'll get in real life because there are tens of factors that will influence "real life" driving. Tyre pressure, outside temp, weight, etc. Plus the driving style of course.
You may think it is a con but it's in your interest to have good official fuel cons figures and lower real life cons then the other way round. VED is based on CO2 after all.
On the other hand if you were led to believe by the salesman that the car will always or easily perform 74mpg than that was a con and you have grounds for complaint.