Also be aware that some of the fullscreen project properties will force an "aspect ration" on your screen, and it will change whether you are previewing it as fullscreen in browser (with or without the url bar/interface for example, in other words when pressing f11 or not).
Moreover you can make sure to really execute at fullscreen using the "Request fullscreen" action from the Browser object.
In the end a capx can help, consider attaching one to one of your posts.