Wes created it on top of the Genesis Sample theme (from scratch). Since its 1140px by default, he decided to just go with that size since thats what a lot of people want now a days. It doesn’t really make a difference for someone like me since I’m mostly on a laptop, but some people use wide monitors like iMacs, or external monitors to browse the web and a narrow site looks too small sometimes. But I think moving to 1140px is a good idea since we can easily revert back to 960px with a single modification.
Glad I could help!
I create awesome sites for awesome people! Contact me if interested – ericsanchez1585@gmail.com