OK.....

[swenocha]

Dave is correct. You'll note in the chocolate bar example, they start and end with 25 squares in the bar (but with one extra square on the side). The diagonals are what get you the extra (but slightly shorter) squares. Not small enough to be noticeable in the box, but exactly small enough to make up the difference of one square.

This image, despite its poor grammar, shows it better than I can explain it...

[evily]

Yes, what Dave and Swen said. They are making a row (or two or three) of the squares slightly shorter. (I like how the image Swen posted says "some become more little". That's one way of putting it!) It's just a trick of the mind/eye.

[mtsoxfan]

OK, who's the witch now... turning blocks into chocolate...