Many reasons, not least the aerofoil. Different aerofoils generate different amounts of lift at different speeds. As a generalisation fatter sections tend to generate their lift at lower speed than thinner ones, however the thinner ones generate less drag at higher speeds, which is why the new generation of fast and slippery glass ships tend to have higher landing speeds than the older ones and require different take-off and landing distances.
Also bear in mind that you are not talking about something that is in a state of equilibrium, the aircraft is actually being accellerated so your horsepower is not only being expended on lifting the thing off the ground (potential energy) it is also building up kinetic energy.
Mike
(Grubbing around in the distant recesses of his memory for the remnants of his O Level Physics)
