Your generalization that "the reflexive pronoun should always match its subject" is faulty. There are different types of pronouns, and not all pronouns take matching reflexive pronouns. The personal pronouns all have corresponding reflexive pronouns. But the indefinite pronouns somebody and someone don't take "somebodyself" or "someoneself" as their reflexive pronouns; they take third-person reflexive pronouns like himself, herself, or themselves/themself.
The indefinite pronoun one often takes oneself as the reflexive pronoun, but you can also find examples of the third-person pronoun himself (mentioned in Fowler).
And as you've described, interrogative pronouns also take third-person reflexive pronouns.