Keep in mind, port arrival time is different than boarding time. PAT is literally when you are scheduled to/'allowed' to arrive at the port. There are several steps you need to do once you arrive, and then you will likely have a wait in the terminal until your boarding group is called. There is a correlation between PAT and boarding group (earlier boarding groups have earlier PATs), but your PAT and your boarding group are two different things.
My parents cruised
DCL for the first time in 20 odd years last September and didn't realize it (and I didn't think to explain it). They thought/assumed that once we got to the port and through security, etc., that we'd walk right on the ship. Waiting is always hard, but doubly harder when you weren't expecting it. We were group 5, I think, and boarded around noon IIRC, so still not bad. But there was a wait between port arrival and boarding.