I think Jane is talking about View options on the Bookshelf page (on right under the green search bar) rather than where the Bookshelf is viewed from. And the reason for the difference is that the Gallery View (the right black squares) only shows books with book covers - it is a visual representation of your Bookshelf. The same is true of the Recipes Gallery View - you only see recipes with photos.
So the reason for the difference of 1 is that you have one book on your Bookshelf without a cover image. I took a look and found it was The Frenchwoman's Kitchen. I have now added a cover, though none available online were very good. So if you would like to take a scan or square-on photo of your book cover and email it to us, I will replace it. But anyway, your numbers will now match.