For a specific negotiation, needs and objectives tend to remain relatively constant. If my objective is to increase market share or sales volume, this will usually serve as the anchor or focal point for the rest of my plan. Setting objectives ensures that you have other alternatives to getting your needs met (e.g., other clients or sources of supply). These objectives would not change if you moved to a negotiation with another party.
A position, by contrast, represents one way to accomplish objectives or satisfy needs. Granted, there are probably preferred positions. However, successful negotiators tend to be more creative and flexible about the positions they take. They do not lose sight of the fact that the ultimate goal is to achieve their objectives, even if that means that they must alter or modify some of their positions.