They WANT it to show that, while mostly knowing long before, that it is sold. 99% of vehicles showing "In Transit" are presold vehicles. Not ALL of them are, but the very vast majority are.
Why would they show in transit anyways? Because they want you to call so they can potentially talk you into something else that they do have available. I called about many "In Transit" vehicles, but by the lucky grace of God, the one I got wasn't sold yet. They had 4 incoming on the carrier and listed on the website, 3 of the 4 were presold 6.2's, the last was a Dmax orphan. But little did they know that was the only one I wanted. Just blind luck really.
I also believe that as soon as they get a VIN they can list it as "In Transit" even though it may take 4-6 weeks or longer to actually get on the ground.