Although the resolution of the screen on the 7" PlayBook is 1024x600, the browser window size is always less.
In the default configuration, the browser viewport is 1024x512 pixels. That's when the back/forward buttons, address field, history/bookmark and fullscreen toggle switch are showing.
I think it would have been nice if "fullscreen" mode on the browser eliminated all window chrome, including the title bar, but unfortunately that's not how they've done it.
With only the title bar showing, your window size is 1024x569 pixels, so the title bar steals 31 of those valuable pixels just to show you the name of the page you're on, as if you didn't know already and couldn't tell easily by doing a top-swipe to see the address field.
I found this site helpful in identifying these numbers, and it also has a nice bit of code (jQuery-based) that looks useful for making a site which can adjust its content to fit the browser window size, which seems especially helpful for mobile devices.
I have some internal-use sites which I will be adapting to better fit the limited resolution of tablets. There was never a reason to with smartphones, as I find them just too tiny to be effective for real work, but after almost a month of use I've definitely concluded that a tablet is a different story, and has serious productivity advantages when used appropriately.