> The same TX & RX are shown for both interfaces.

That implies that they are layer 2 counters and will therefore be the same for a given physical interface.

> I don't know whether the tools have changed much since those I'm using on this box, but the difference *might* be that Grant's IPs are on the same subnet. I *think* it's possible for the route (as in: `man route`) to configure packets that come in on 1.2.3.2 to be replied to via 1.2.3.1.

If the return packets are part of the same TCP session it will automatically be sent from the correct IP address. If it didnt, the remote end wouldnt consider the return packets part of the same session and reject them.