In short, horse trading. That is, if you have leverage in an adversarial negotiation, you'd be foolish not to use it to get more of what you want.

For example, as a prospective employee, if I knew that I was the only qualified candidate the employer had interviewed, and they really needed someone within a week, I'd know that I can ask for more salary. If I instead take the middle of the salary range, just because it's maximally acceptable to both parties, I'd be missing out