Sorry, I have missed your post yesterday.
The background shift is made to synchronize background fill position relative to the page design during page centering and scrolling. In some cases users may want them match perfectly.
It's done with a help of script. There's a rx() function assigned to the window.onscroll event. It does some tweaking for the rollover mechanisms, but it also adjusts background fill position based on current scroll and page position.
The fill offset is set this way:
document.body.style.backgroundPosition=dx+'px 0px'
Where dx is a calculated required offset. Look into that function or ask me for more details.
Hope it helps.
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