it's just some colloquial bullshit created by the common misunderstanding of the phrase "exception that proves the rule".

that phrase actually is supposed to refer to a rule that isn't written explicitly but rather must be inferred by some statement in the negative. for example, a sign that says "no skateboarding on weekdays" means you can skateboard here on the weekends, a rule which we can understand even though we only are given the exception to it