An Actual Fact (out of character for Usenet, but don't tell anybody) from a
friend who's an airport manager. I posed the group question, and his reply is:

"Most airports have that extra foot for insurance reasons. Aircraft insurance
policies for GA and corporate aircraft limit the operators to runways longer
than 3600 feet, 4000 feet, 5000 feet or whatever. So we build runways one foot
longer than what we think the insurance carriers require. The other requirement
that insurance carriers what are precision approaches. Aircraft owners of
turbines can get reduced premiums or are limited to use runways with precision
approaches."