Manufacturer's fuel consumption figures were always meant to be a simple tool to show how different models compared. It wasn't meant to be a real world figure attainable by the average motorist. Tests were conducted on rolling roads inside a climate controlled room with a computer program applying the throttle etc. All manufacturers tested their model in exactly the same way. In fact I think they had to submit vehicles to an independent testing body.
A couple of years ago the way these tests were conducted was changed to randomly selected samples from production being fitted with recording equipment and being done on the open roads. The results from these new tests were very much more in accordance with real world figures.
But the bottom line is everyday fuel consumption is easily affected by many factors beyond the control of the manufacturer, not least the way it is driven!