Canvas buffering screen size

Question

I wrote these code in a class which extends Canvas. (please have a look at the pictures at these links if you don't want to read, i can't post pictures directly because of stackoverflow reputation or something) http://tinypic.com/r/oaw3u8/5 http://tinypic.com/r/24g9ldz/5

    final static int WIDTH = 800;
    final static int HEIGHT = 600;
    Dimension SIZE = new Dimension(WIDTH, HEIGHT);

    Game() {
    setPreferredSize(SIZE);
    **
    **
    *
    *
}


    public void render() {
    BufferStrategy bs = getBufferStrategy();
    if (bs == null) {
        createBufferStrategy(3);
        return;
    }
    Graphics g = bs.getDrawGraphics();
    g.setColor(Color.cyan);
    Graphics2D g2 = (Graphics2D) g;
**g.fillRect(0, 0, getWidth(), getHeight());**
    g.dispose();
    bs.show();
}

question is about the double-stared line of code, which is used for refreshing the screen by drawing a screen-size rectangle. **at the first time, my code was g.fillRect(0,0,WIDTH,HEIGHT);
then i notice the balck rectangle was smaller then the screen. there was a minor,about 1 inch, margin at the right inside of the frame.(i might use "margin" wrong but i hope you get what i mean) so i used getWidth(),getHeight() instead of using the exact variable WIDTH and HEIGHT. and it worked well,
i don't know the reason why ? please help me why is this happening, are variable not accurate in this situation? or what . thank you very much