Somehow this pax got past the gate staff as well as the cabin crew who check boarding passes at the doors. I am sure that many things have changed since I was airport staff but in those days we had to keep the stub of the boarding pass and keep them in piles for 20 so that we could count the boarded pax if the system went down (often). If the flight was full then you might even get a double seating error at the gates.
WRT the cabin crew at the door, distracted maybe?