User blog comment:Rupert Giles/Layout Changes: Breakpoints and Typography/@comment--20150417041233/@comment-452-20150418000856

I just overlaid Suminoma's screenshots, and the background is definitely narrower, so you might want to look into that.

edit: Suminoma's screenshots are different widths, so perhaps what is shown in them is deceptive, but I tried to check the images in the blog posts, and they are also different widths. If these images are taken at face value, they also show the background image being narrower.
 * All of the "before" images are 2300px wide
 * All of the "after" images are 2000px wide

edit: Suminoma's screenshots are only different widths because the scrollbar is cropped out of one but not the other. To make the left background align, I had to move the "after" image 221px to the left.