Hi BRL,
Yep, it's just the width of the web page, conveniently not set up automatically to be the same size as the paper..
Click on File | Print Preview and click the settings button. (shoud be just in from the left at the top; a doc icon with a gear on it) change the left and right margin values to zero (just double click the value and type 0). When asked if you want to fix these prior to printing because they are outside the printable margins click no.
That should do it.
Cheers
Liam