Actually, it varies based on authority cited. Plus the moisture content of the flour, on any particular day, can affect the average weight.
For what it's worth, Peter Reinhart uses 4.5 ounces as the standard equivilent of one cup. Because much of my bread baking is based on his methods, I've adopted that figure.
You can accept any of the published figures. Or, a better way, is to weigh ten cups of flour, then figure the average and use that as your standard when substituting weight in recipes that use cups.
They have taken the oath of the brother in blood, in leavened bread and salt. Rudyard Kipling