How can this be true?

As you can see in the upper arrangement, we have put 2 triangles and 2 other shapes together in a "big" traingle ABC. This triangle is 13 X 5.
Now in the lower arrangements, we have the same 2 traingles and the same other 2 shapes, rearreanged in the SAME 13 X 5 big triangle ABC. BUT it looks like ONE SQUARE is not covered !!! HOW CAN THIS BE TRUE ???
Hint: I will disclose the cunniing brain manipulation next week, 4 the 1's that couldn't figure out.