A13/B13 are the same gate. This arm of T3 is used for both US and domestic departures. When used for US departures, the gate designation starts with A, but for arrivals and domestic departures, it is designated B. I don't believe gate 13 is ever used for domestic departures, but B17 through B19 can swing between US and domestic departures, and would be indicated as A17..A19 in that case.