The angles of the dark green triangle and the red triangle are not the same.

Note that the hypoteneuse of the top "triangle" is not a straight line. It dips down then up. The area lost by the dip down equals the area of the white box in the lower triangle.

The top "triangle" dips down because the smallest angle in the red triangle is smaller than the smallest angle in the dark green triangle.

My answer is not very technical but it clearly is the answer.