Originally Posted by
Zfz
This is because you are getting a different group of people each time.
The matching system works in a two-stage process.
Stage 1: form a party from each of the role queues. Once the party is formed, send them to stage 2.
Stage 2: allocate instance, then pop the Commence dialog.
When someone withdraws, you restart from stage one, from scratch. That means the party is completely dissolved, everyone (except the one who withdrew) is thrown back to their respective role queues, and the system starts forming you into a new party as if you've never went past stage 1 in the first place.
This process is also why you can pop the Commence dialog (meaning you have been successfully allocated an instance), then someone withdraws, and then you form a new party only to sit at the Server Reservation.
It's also why at odd hours when there's only 1 tank in the world queueing dungeons, you could have 1 DD timeout (i.e. 1/1, 1/1, 1/2 at the Commence dialog) and then go back to staring at 0/1 1/1 2/2. It's because that only tank got sent to a different party.