If you check the source of the page, you will see that the "Content Pane" is coded to take 67% of the page width.  

The "right pane" is coded to take 33% of the width.  If the "left pane" had content it would also try to take 33% of the page width.  The expectation is that site admins will want to use 2 of the 3 panes, not all 3 at once.  The unused pane will not take up any width.