Although the images are duplicates, one has the impression that the tower on the right leans more, as if photographed from a different angle.
The illusion was discovered by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu at McGill University, and won first prize in the Best Illusion of the Year Contest 2007.
[1] The authors suggest that the illusion occurs because of the way the visual system takes into account perspective.
[2][3] When two identical towers rise in parallel but are viewed from below, their corresponding outlines converge in the retinal image due to perspective.
The illusion reveals that the visual system is obliged to treat the two images as part of the same scene, in other words as the "Twin Towers of Pisa".