People typically consider “win-win” as interchangeable with “ideal.” Really, though, a winning outcome is one that’s best among your alternatives.
So, finality could put you in the win column when the offer outweighs the alternative of protracted (costly) litigation and the possibility/necessity of appeal. Certainty may, too, if the proposal beats the risks inherent in a decision-by-another. Immediacy has its advantages, as well.
Intangibles should also be factored in. Decisions in mediation often involve quality-of-life decisions for yourself, your family, your workforce, etc. They are personal judgment calls. Your win may be the result that leaves you on your strongest, most capable footing.
The fact is, you dramatically increase your odds of achieving a win at the bargaining table when you embrace the notion that “better than,” rather than “best possible,” is a worthy aim.