I may be over-simplifying it but it looks to me, as if it's constructed like a book. You'll see what I mean, if you take two sheets of paper, place them together and then fold them down the middle. Now you have four pages, each with a front and a back. If you open the 'book' flat, you'll see the front and back covers (pages 1 & 8) share one sheet side of paper. The two middle 'pages' (4 & 5) also share one sheet side. BUT if you open it to pages 2 & 3 or 6 & 7, you'll see that each of those pages shares half of one sheet and half of the another.
I hope that makes sense. It's after midnight here and I need to be in bed. LOL