Wide screen monitors are too wide to display a web page so splitting the screen in half to display two things side by side is very common - that makes 960 a very common browser width including the browser chrome.
I never display web pages wider than 950 (half the width of the screen less the scrollbar) and many other people do the same.It is one sie you MUST check your page displays correctly at if you don’t want lots of people thinking your site is broken.