The reason changed as one is Fixed and the other is Flexible.

If you want flexible frames, because of monitor sizes. If you use the one with the container at 90% width, you have to add a min and max width. If you don't you will get scrollbars appearing on some pages. I prefer to use the fixed frame settings. Then you can set width to a fixed 960px and you can have then a border setting of what you like, here it's at 9px solid and black.