Mr R-D and Miss Emma's Web World HMS Challenge

So you're good at maths are you?
Well have a go at the little teaser below and then e-mail us the answer (there's a button for that below).
This one is passed on from MJH, JP, and WRD

In the rectangle on the left, the area is found by multiplying one side of the rectangle by the other as follows:

8cm x 21cm = 168 sq cm ok?

The shape on the right has been made from EXACTLY the same pieces from the shape on the left, but this time arranged to make a square. The area of this square is found by multiplying one side of the square by the other as follows:

13cm x 13cm = 169 sq cm ok?

The question is, where on earth did the extra 1 sq cm come from in the shape on the right?

[Hint: it may help to make two 'a' pieces and two 'b' pieces on graph paper first]

[If you really have tried and just can't work it out, the answer has generously been supplied by JP of HMS. But surely, you must have a go though!]