Take a careful look at the upper figure: it's not a perfect triangle (the part where the blue starts slopes up more steeply than the rest). Same goes for the lower figure really. So, by smuggling tiny little bits here and there, extra surface area can be created to allow for a free black square.

The arrangement of the black lines isn't similar across the two figures either.

Just to illustrate:





I've highlighted two obvious differences.

See?