The width across the screen was what the lady who created the theme (roulette) chose to give it.
In the CSS, you can set the whole width to a percentage of the full screen (I think October's 80%), or different sections (Default has the header and footer reduced to a smaller percentage than the main content div). It makes it easy to adjust widths because all you have to do is change the percentage (if you take that approach). Others like widescreen are close to 100% with just enough space that you can see the background a little bit each side.