It's not impossible, because the sun's relative position changes, and the closer to orthogonal the light, the better the efficiency of the solar cell. By taking optimal advantage of the height, this design is closer to orthogonality more of the time, and is thus more efficient per ground area (though not per solar cell area, as you demonstrate).

Now, if your solar array were mounted such that it actively maintained orthogonality to the sun (heliotropism), I expect you'd do even better, but that kind of active system is more subject to failure.