All is well. The top "triangle" is not a triangle at all, but rather, a four-sided shape.

The hypotenuse (slope) is not a straight line.

If you take the angle between the hypotenuse and bottom of the green triangle: invtan(2/5) = 21.801 degrees.

And of the red triangle: invtan(3/8) = 20.556 degrees.

This would explain the difference in area as we cannot use the equation, Area = 0.5*length*base.

Daz: I am no longer offering you money for your solution.