Diffraction creates patterns (of light distribution) which depend on the shape of the aperture (which is why in the printing industry, making half tone plates, they used a choice of shapes). The Airy disc may be a triangle lozenge, pentagon, or whatever form, and those forms wil create uneven 'density' in the pattern, distinct directions.
So, as Hikari already mentioned, the shape of the aperture may have an effect.