The average is a generic concept covering multiple more specific concepts like mean and median. If you say something about the generic concept it should not depend on any properties of just one of the specific concepts, in order to hold generally.
Your brother is a term for a single person that is simply under-determined and could turn out to apply to either one, but not both. What you say about your brother should apply to the brother you mean, in order to hold.
The average is a generic concept covering multiple more specific concepts like mean and median. If you say something about the generic concept it should not depend on any properties of just one of the specific concepts, in order to hold generally.
Your brother is a term for a single person that is simply under-determined and could turn out to apply to either one, but not both. What you say about your brother should apply to the brother you mean, in order to hold.