As a normal part of the bookbinding process, a binder trims each edge of a page so that the assembled book fits together smoothly.
Whenever you order an iPhoto book with full bleed images (those that go to the very edge of a page), the final bound image may contain a little more or less of the original image at any given edge than what you saw on your screen. For example, you may lose a portion of your image at the spine (known as the "gutter"), while gaining a little bit at the outside edge. Though iPhoto approximates the final output as much as possible, the normal margin of error in the publishing process precludes a 100 percent predictable result.
For this reason, when you prepare your images for an iPhoto book, you should err toward cropping photos a little bit loose rather than cropping in tight. For images that you've shot very tightly in camera (those that won't look good with trimming), select page designs that aren't full bleed.
As you lay out your book, always stop to think about how a page may get trimmed for a moment, and edit your images accordingly—you'll be happier with the results when you see the finished product.