Extra Large Curry Triangle and similar items

The LARGE Curry triangle, also sometimes called the missing square puzzle. Two triangles the same height and the same width at the base, have exactly the same sized pieces of various coloured acrylic and yet on the left hand side somehow we have managed to gain 18 square centimetres. How can this be?? Made from 2 layers of 3 mm thick white acrylic. Overall size is 600 mm long by 400 mm by 6 mm thick