I'm quite confident that the definite answer is that Braviary gives out 2 Attack EVs, so Game Freak wanted Mandibuzz to give out 2 Special Attack EVs to keep the counterpart trend going. Game Freak does this kind of thing with a lot of counterpart or duo Pokemon, and they will choose consistency over logic in situations like this one.
Of course, we aren't Game Freak so we don't know for sure, but given how Game Freak likes their opposites, I think this is an accurate assumption.
