The highlighted guide lines reveal the counterintuitive geometry of the two tables. The two blue lines are the same length, as are the two yellow lines; hold a reference object up to the screen if you don't believe it.
Beau Lotto theorizes that the reason humans are vulnerable to optical illusions is that we are not really responding to the physical properties of the object we are looking at. Rather, we "see" things based on what other things similar to the current image have typically turned out to be in the past.
The implication: The visual cortex is evolutionarily adapted to the sensory information it receives, not to the world beyond. Such forced interpretations allow us to recognize important objects (fruit in a tree, a tiger in the grass) quickly, but at the cost of total accuracy. Basically, we do not see the world the way it really is because our brains will not let us.