You may notice that sometimes when you turn pages, the page number doesn't change. Since the screen on the NOOK can contain fewer words than those of a physical page of a book (depending on the font size being used), sometimes it will take several screens of text to display an entire page. In these cases, the page number will not change, although you are making progress through the book.
Also, books are formatted differently for digital reading than for printing, which may result in different numbering.