This means that adding pages is akin to adding at least one leaf at a time i.e. two pages to the book. Click on the + button on the editor toolbar and it will add two fresh pages at the spot where ActivePage had been.
This is a unique behavior that aims to enforce "evenality" of a book, something that has not been experienced much in the world of software.
With $ bookiza insert [page_no] on the terminal, it is even easier to manage than on the browser.