The expression is often used to mean that the exception to the rule means that the rule is true. Now, that is not the original meaning (but hey, language lives and flows). The original meaning is more along the lines of “by stating only the exception of the rule, I am implying the rule”. Wikipedia gives this example: a sign “parking prohibited on Sundays” implies that on the other days of the week, parking is allowed.
Which is why I think I cringe when I read articles talking about “women directors” or “female whatever”. It implies that the common, “normal”, the standard is male and maleness. Which it *is* in some professions. I am convinced however that life shaped language and vice versa. When you keep talking about women as if they are the exception (and not the majority of humans), you make it that tiny bit harder to find acceptance for them. When I read articles about male directors/singers/artists/whatever, I would be very surprised to find questions like “and as a man, how did you …”, because mostly those articles deal with the achievements, the character, the likes and dislikes and what have you of the interviewed. Often their sex isn’t mentioned. Because it is “the norm”, “the standard”.
Please, journalists, next time you write up an interview with a female interviewee, think about how it would sound to you if you wrote “the man director” or “the male singer”, before you phrase similar things about women. We are not an exception.