Because magnets have two poles, the force drops off by varying amounts, depending on the shape and size of the magnet. (The closer pole attracts, while the farther one repels. The other magnet also has two poles, which makes things even more complicated. For example in a short magnet the second pole has a greater influence than it does in a long one.)

It also depends on if you are interacting with another magnet, or with unmagnetized iron. ^3 or ^5 is just an approximation - it can go to ^7, and it's not a definite number, it varies.

See: http://www.exo.net/~pauld/activities/magnetism/forcebetweenm...