pronouns

I am interested and confused by the social convention of when people include their pronouns in their bio: many people will include (he/him/his) - three pronouns. To me, it seems that him, and especially his, are just derivative of the subject pronoun he. It would be sufficient to say "pronoun: he" and the reader could infer the rest.

The only time I don't see a "he/him/his", "she/her/hers", or "they/them/their" is when someone says "he/they" or "she/they". In those scenarios, they are actually providing two subject pronouns (like saying "he/she", as opposed to a subject pronoun and its corresponding object pronoun like "he/his" or "they/them") expressing what I imagine to be a flexibility between two pronouns or gender identities, but also inferring the object and possessive pronouns by only providing subject pronouns.

I guess the entire convention of pronouns in the bio isn't really coming from informed linguists or grammarians, so maybe it's natural there's some redundancy and inconsistency among participants. People see he/they and think they should also have two, so they say he/him, even though that's not the same thing at all.

I wonder if anyone really ever prefers an object pronoun or possessive pronoun that doesn't correspond to its subject pronoun? Is anyone's pronouns "he/her/theirs"? I can't see why that would ever be the case, no pun intended.