They use company wide averaging like you said. They also look at the industry you work in and the type of work you do. An engineering firm that does most of its work on a computer is going to have a cheaper plan for the exact same coverages than an industrial firm whose employees work on heavy objects and in tight spaces.
Why cant you do the same for the entire country? Differences in the cost of health care in different regions is one. individual under writing would be too expensive is another.