The Appearing Square

Can you explain where the extra square comes from?

This puzzle is based on the illusion that the shape shown in both cases is a triangle. In fact, the 'hypotenuse' is not a straight line in either case - the red and blue triangles have slightly differing acute angles and the 'spare' square in the second example is the result of the bowing out of the longest side in this case, as opposed to a bowing in in the first example.