There is a very good rational for 940px, although it might be a bit historical at this point. It was, as you guessed, not Weaver’s number, but came from the Twenty Ten theme.
It is based on 1024px width, which was perhaps the most common screen width, and still is for cheap notebooks and some tablets.
But the theme width is the width of the page viewport within a browser, and does not include the browser borders and vertical scroll bar. It was found that the 84px left over from 940 would allow most browser to fit without a horizontal scroll bar on 1024px screens.