1) Because the electric system gets charged for free by regenerative braking for some of its power and that free power is used to augment the dino-fuel engine so it burns less petrol in accelerating
2) Because the MPG test allows the car to start with one level of charge and finish with another. Naturally, manufacturers game that allowance to their advantage.
Spoke to a fellow recently with a rare 1st generation hybrid Honda Insight who get 80mpg on a run. Of course though the Honda Insight had "All the utility of a sports car with none of the performance"