The differential in distance is greater from side to side than it is from top to bottom. In other words, the distance between the control lines on the left and right is greater than the distance between the forward and reverse control lines. This allows a more efficient angle of attack. You can hold an upright hover in the same wind speed if you bring in the top lines toward the handles, but you would lose control ability in other aspects of flight. As always, it is a compromise of adjustments that allow optimum performance. You can't have all the extremes at once, and once you go beyond any extreme flight becomes impossible and the kite falls from the sky. The surface area of the airfoil doesn't change, but its shape and orientation to the wind flow do.