A more thorough explanation:

Term: ex facto
Definition: Ex facto means "from a fact" in Latin. It refers to something that is true or exists because of a fact or action. It is the same as saying "actually" or "de facto".
Examples: The ex facto leader of the group was the person who had the most experience. This means that the person who had the most experience was the actual leader, even if they were not officially appointed. Another example is if a company has a policy of not hiring smokers, but they hire someone who smokes anyway. That person is the ex facto exception to the policy because they were hired despite the policy. These examples show how something can be true or exist because of a fact or action, even if it goes against official rules or policies.