Do you have an extra toolbar installed?
This will make your viewport smaller,hence the scrollbar

I use firefox with a webdevelopment-bar( http://chrispederick.com/work/web-developer/ )that shows the exact size of your viewport and you can try to accommodate to it
But you'll never know exact what your viewers will be using,like;
-extra toolbar like google
-panels open(history,favorites,rss)
-or even changed resolution for larger display

Hans