It seems like the gaps should appear on both sides. I recall reading something about settings that cause the layout to be drawn from the upper left, but of course I can't find it now.
I was re-reading through this tutorial and it sure sounds like it's an issue of aspect ratio: (Other than the gap is on one side.)
[quote:143056tt]This is the same problem faced by TV producers. There are many TVs out there using aspect ratios of 4:3, 16:9 and 16:10. If a producer films a TV show only in 4:3, on a 16:9 TV there will be gaps at the sides, or possibly even unintended off-set equipment and crew visible! Similarly, if you draw a background exactly fitting one display, then run it with Scale outer mode on another display with a slightly different aspect ratio, gaps will appear at the side or content outside the layout becomes visible. This can even happen if you simply don't take in to account the space taken up by the status bar on devices like the iPhone.
There are two ways to solve this problem:
1) Use Scale outer mode, and draw your backgrounds wider (or taller, depending on orientation) than the window size, past the normally viewable edges, to ensure no gaps ever appear regardless of the device aspect ratio.
2) Use Scale inner mode, and make sure nothing important is close to the edges, since the edges are susceptible to being cut off on different size displays.
You might try the solutions offered just to see if it changes anything.