If behaviour is excusable, people can forgive it. You still did something less than good, but there is a reason others can understand — you were ill, you were new, you did not know the rule. It is softer than right or justified. Justified says the action was fair; excusable says it was not really fair, but we will not be hard on you. Teachers, parents, and rule-books use it a lot: “a mistake, but an excusable one.” The opposite, inexcusable, is much sharper: there is no acceptable reason. Though actually, what counts as excusable changes with the room you are in — a slip that is excusable at a family dinner may not be excusable at work.