Rhys, think of it this way, as you get to higher and higher speeds, a bigger percentage of the cars power is used up fighting to get the air out of the way and far less of it to accelerate the car.

So if 2 cars have the same power curve and gearing and 1 car is a little heavier but more aerodynamic, then at low speeds it will accelerate slower but at higher speeds it will acclerate more quickly.