because in one orientation the hypotenuse is sticking up in the middle and in the other orientation it is sticking down in the middle. In other words one has squashed it up. The squashed up one doesn't have a gap as the area of the gap is lost where it sticks up.
I know that is a really bad explanation, try cutting them out, after all i only managed a D at a-level maths, i was going to revise but i spent the day fitting a sub box instead. Then i went onto an engineering course and had to work like **** to learn what i was supposed to know. How was i to know it wasn't all useless :/