That seems to be a byproduct of having the top ad in the logo strip table which contains everything above the bar with the "The Allspark Homepage" link.
The logo strip table uses
http://www.allspark.com/test/header_full.jpg as the background, with the image right-aligned so the "WE ARE ONE" text is always displayed on the page.
When the browser window width is decreased, the logo strip table changes size to match by decreasing the width of the ad column. When the ad column reaches it's minimum width (that is, the width of the ad), the logo strip cannot get any smaller, forcing the right side of the table off-screen unless you scroll to the left. Since the "All Are One" text is part of the background image it starts from the right side of the table regardless of the table's width.