could you take a couple of screens? 1 of the iWeb preview, and another in your browser. Try publishing in different browsers too, as not all features will work in a single browser. Just be sure too that the dimensions are appropriate 800x600 (or slightly less, taking tabs and menus into account) is still a safe way to go.
Haven't used iWeb at all (I design all my own work, and templates bug me) but does it have some kind of browser check? A feature like this checks that the pages you create are as web friendly as possible.
Also, have you uploaded it? If so, could you give us a link. It might just be your browser or browser settings.