I *think* it shows the flaps regardless of what kind of book you buy. When I made our softcover reception books, it showed the flaps even though softcover books don't have flaps. The software doesn't know what type of book you're buying.
Also, I have heard there were problems with the latest version of Blurb's bookmaking software - I haven't downloaded it yet cuz I wanna wait and see...
If your book comes and it's wrong - contact customer service. They are really responsive. They replaced my engagement book for free twice when there were printing problems with the cover.