It goes in a question and answer format something like: "You knew a girl in high-school. In situation A, you find out years later that she is a stage actor and a dancer. In situation B, you find out years later that she is a stage actor. Compare how likely the second one is to the first one" In plain English there's an implied meaning of "she is a stage actor and not a dancer" in situation B. But that is not the statement. Situation B is as likely or more than Situation A but not if you use the English language description which suddenly is under-specified without real-world information.

Obviously you pick the things to play to the priors of the person you're asking so that if they think actors frequently dance you put that in there. That leads them down the false rabbit hole of whether it's "stage actor and dancer" vs. "stage actor and not dancer" that you're comparing.

In pragmatics (a subfield of linguistics) this is called "conversational implicature" - in particular it's an example of the "maxim of quantity".

Which is to say, you are correct about how the language works and mathematicians are a little weird.