The reason for magnetic field strength falling off as 1/r^3 is interesting: the biot-savart law says that magnetic field falls off as 1/r^2 from a magnetic source, but in reality sources tend to be better approximated by magnetic dipoles than magnetic monopoles. A "north pole" is always accompanied by a "south pole", and at distance there are "interaction effects" such that a part of the field strength is "canceled out".
