Fairness is difficult because equal treatment and equal outcomes are not the same promise.

Two different promises hide under the same word. Treating people identically ignores that they started in different places; aiming at identical outcomes means treating people differently on purpose. Both are defensible, and most arguments about fairness are really arguments about which promise is being made, with each side certain the other is being unjust.

In your next disagreement about what is fair, ask which version each person means. Naming it does not settle the question, but it usually turns a moral standoff into something you can actually discuss.