You've specified a nominal load that is small relative to the size of the room. The way the nominal load feature works is by artificially shrinking (or, in your case, enlarging) the occupant-occupant collision radius of occupants when they enter the elevator room. It works very well when the radius changes a little and very poorly when the radius increases a lot. In your model the first person walks in and he doesn't know to go to the corner opposite the door - leaving his hugely inflated bulk blocking the door to go in.

At the moment, Pathfinder only option to limit the number of people in a room is the collision radius based nominal load. If you want three people in the airlock, your best option is reduce the area of the room either directly or by adding some furniture. If pathfinder calculates that a radius factor of 1.0 will fit 3.4 occupants and you ask for 3.0, the resulting radius factor of 1.13 will work a lot better than the much bigger factor you are getting now.