The Missing Piece

Below the four parts have been reorganized. The four partitions are exactly the same in both arrangements. Why is there a hole?

Hint : 
         This is not an optical illusion. If you print out the puzzle, cut out the pieces and rearrange them, you get the exact same dilemma.

Solution : 
         The gradient of the teal hypotenuse is different than the gradient of the red hypotenuse.