When we compare the two triangles, they appear identical to us. A fast inspection, helped by the grid, shows us that we deal with two triangles of 13 squares of width and 5 squares height.

In fact the higher triangle is not a triangle at all, it is a tetrahedron. If you look at attentively, the hypothenuse (the diagonal), you will see that it is not completely straight, it is a little bit concave. The lower triangle is not a triangle either, it is also a tetrahedron, but this time, the hypothenuse is a little bit convex, beacause the position of the red and green triangles is now reversed.

The two figures are composed of the same pieces. But because the "lower triangle" has a hypothenuse which is convex, the additional empty surface is the equivalent of a square, which explains why the lower triangle has an empty box.