Sometimes, an author needs to make updates to their book. This is normal, and in many cases, the changes can be made to a book while still using the original ISBN number. (There are cases where a new ISBN number is required. For more information on this, click here.)
While these updates are made, the prior version is still available for purchase within the market place. If retailers placed orders for your book prior to the update, those orders were printed with the original layout. Any orders received after the update will use the new layout. When retailers receive new copies, they are put into inventory with any copies already in stock. When an order is received, the copy shipped to the customer could be the newer or older version. This means it may take several weeks for retailers to flush out old copies with the original layout.