It's the so called offset of the document, every browser uses special values. But you can elimate this by pressing
[Ctrl]+[J], here you can set the document borders to zero. I don't know what there called in the English (or Danish?) version of Dreamweaver, so please have a look on the image I attached.
Of course you can adjust the borders within the HTML source. Search for the <body>-Tag and edit it like this:
Code:
<body leftmargin="0" topmargin="0" marginwidth="0" marginheight="0">