My first guess wouldn't be routing, but traffic-shaping.
Perhaps the VPN you use is on a protocol/port that isn't outright rate-limited and since ATT can't peak inside your tunnel to see what you are doing with the bandwidth, it avoids any QoS/shaping/limiting that your non-VPN connection is subjected to.
Perhaps the VPN you use is on a protocol/port that isn't outright rate-limited and since ATT can't peak inside your tunnel to see what you are doing with the bandwidth, it avoids any QoS/shaping/limiting that your non-VPN connection is subjected to.