You nailed it. Only makes sense. It is a balance for not allowing a black man on the other side of the party slate, and, it balances having a woman at the head of the other side. That shows the United States voters that it really is not that odd having a woman run for this office.
That way, when Hillary is crowned, everything will look more or less 'kosher.' I mean, sure, there will be a few around like those kooks at 4um talking about votescam and such.... but the sheeple will buy it.