I just want to mention that there can be legitimate reasons for someone to not be on their original DME bus.
Our youngest DD uses a wheelchair and can't board the DME bus without having a lift bus. In the early days of DME , our appointed bus came but had no lift. This was even though a lift bus was specified on all our paperwork, we had verified the need when we checked in at the airport and reverified curing our stay. When our bus showed up without a lift, the driver verified it was the bus we were scheduled for, but without the lift that was on the paperwork. The driver called and a different bus was diverted to pick us up.
On another trip, our assigned bus did have a lift, but it became disabled on the stop before ours. So, another bus was diverted to pick us up again.
Because of that, our bus is now scheduled extra early.
On our last trip, we were at the bus area about 30-40 minutes before our bus departure time and we saw 3 buses arrive before ours
(we checked with each driver to make sure it wasnt ours).
We saw one group who missed their bus because they said they thought the departure time was the time they were supposed to get on the bus. That driver did let them on after checking his list and seeing he had extra room. Our actual bus was very full - like almost all seats taken. As he was loading DD, he told us that the reason his bus was so full was that people for the next bus had loaded their luggage on his bus while he was busy with the paperwork of another guest. By the time he checked their paperwork and found they belonged on the next bus, their luggage was buried behind other people who did belong on his bus. There was no way he could remove their luggage without doing a lot of unloading and reloading. He actually had another stop that he was supposed to make, but didn't have room for. So the bus scheduled to pick up the people who boarded an earlier bus was diverted to make his next stop.