Nowadays this problem appears only in FF (I guess) - The bug appears when trying to align even numbered width backgrounds, with an odd numbered width browser viewport (or vice versa). For example, if the background is an even width of 970px, centering this in a browser viewport of 1501px can have buggy effects on where exactly to center align. This will result in causing this 1px unalignment bug. - Source

Best way is decrease/increase the resolution about 1px.

More about this http://www.sohtanaka.com/web-design/1px-background-alignment-bug/