If we leave the question of math and biology out of it, we can view it linguistically. 

Various theories in pragmatics, a branch of linguistics, have a theory of effective communication that started with Grice's conversational maxims, a set of observations about how people communicate. These are not laws on how to speak, but rather observations on how people communicate. From these maxims, you can posit that people will usually say the most they can to describe their point, but no more. The result of this supposition is that people usually assume that the conversation has followed these Gricean patterns.
For example, let's say I describe an actor as an Oscar nominee. This is %100 true even if that actor actually won the Oscar, but our Gricean assumptions say that if he had won, he would have been described as an Oscar winner not a nominee.

How is all this blather relevant, then? If someone says that they have two children, and one of them is a girl, we assume that they mean that only one of them is a girl, even though the sentence would be %100 true if they had two girls - just because people don't speak in a rational, logical manner - they speak in patterns that evolved to maximize information delivery while minimizing cognitive processing complexity.