The pieces with the same color in left figure have the same size as in the ones in the right figure. In the right image the pieces are only ordered differently. How to explain the hole in the right image?

You are probably being confused by the hypotenuse of the blue and the green piece. It seems like that they together form a straight line. But that is not true, because when you move one box to the right, the hypotenuse of the blue piece will go up 3/8 = 0.375 box, while the green piece will go up 2/5 = 0.4 box. So the hypothenuse of the green piece is actually somewhat steeper than the hypothenuse of the blue piece. That causes the hypothenuse of the left figure to be somewhat bend inside, while the hypothenuse of the right figure is a little bit bend outside. That explains the difference of one box.