When looking at fuel consumption, there is always a lot of "fudge" factor involved which makes getting accurate results difficult. As the saying goes: "your mileage may vary"
Following are just some of the fudge factors that will lead to inaccurate fuel economy measurements and differences between actual and advertised economy. These are all very small sources of inaccuracy, but they add up.
There is really no good way to accurately know the actual amount of fuel used without adding calibrated metering equipment. Unless you own your own fueling station, you cannot control when the pump shuts off, what the temperature and volume of the fuel is, and even which pump you use from time to time. It is also impossible to tell how much fuel was left in the tank unless you run it completely dry (which isn't a great idea for lots of reasons :)) so you're left with a rough approximation.
The amount of fuel used is also calculated by the car, but that generally relies on a calculated value from fuel injector duty cycle and an assumed amount amount of fuel per pulse. This will be slightly off due to the type/quality of fuel, fuel pressure, cleanliness of injectors, electrical and fuel line wear, etc. So that, also, is only going to be an approximation.
Here, again, we are dealing with calculated values based on assumptions. Your car calculates how far it has been based on measured revolutions of the wheels, but it uses assumed values for tire size, tire inflation, etc. so it is also a rough approximation. If you run a non-standard tire size or run your tires in a very under- or over-inflated state, the numbers will be slightly off. Once again - small variation, but adds up over time.
The above variations only address the difference between your measured fuel economy, and the car's displayed fuel economy. Below is the big wildcard, which accounts for the wide differences between your actual fuel economy and what the carmaker says the car will do.
Your driving style and location
Simple. If you drive in very hilly areas, your economy will generally be worse than if you do all your driving in flatlands. Similarly, if you stop and go a lot, your economy will be worse than if you start once and cruise for long periods. Heating/cooling load and electrical load also contribute.
If you are lucky with location and careful with driving style, you can make a Mustang get 40 MPG and by the same token, if you wish, you can make a Prius get 15. Advertised values are based on standardized test cycles that only slightly approximate the real world.
Finally, those are the only factors for cars with a single source of motive force. Yours has another - the electrical motor. The amount of drag or boost provided by the hybrid system is another factor that adds imprecision.
All that boils down to say that advertised fuel economy is really only a very rough estimate of what the car can/will do.