The Missing Unit

I first saw this on Nooey.



Like I said on his blog, this is seems like it should be impossible. You have four static shapes that form a larger shape. They have a set area and so does the large shape. Yet when you move the smaller one's around and form the same large shape again in a different configuration, it seems you lose a square unit of area, yet none of the static shapes lost any area. If anyone can explain please do.

update: The answer is explained very nicely using flash at this site. The puzzle provided some good brain exercise.