The lines are not aligned with the perspective which creates a distorted perception of volume and proportions. Cube 3 is perceived as larger than 1 and 2 but should be the same size.

What is perspective?
Perspective in art usually refers to the representation of three-dimensional objects or spaces in two-dimensional artworks. Artists use perspective techniques to create a realistic impression of depth, and ‘play with’ perspective to present dramatic or disorientating images.