That was my first thought also. But when I checked it on the demo, to make some screen shots to show that, I found that it is NOT the case.
Here's a screen shot of the demo on demo.abantecart.com It is on a Firefox 23" monitor at full screen. You should notice the left and right borders on the banner blocks in relations to the products displayed.
I would suggest this is a coding result and not a screen resolution issue.
Lee