What Jim_B said is right. On several occasions, I've been told by UA folks that what you can see on the reservations screens (Apollo and Sabre, for example) are not the physical seats themselves. So while looking at them give you a general look at the flight, it by no means reflects reality in the way that we'd all like it to... On the other hand, sometimes a full-looking flight isn't as full as it looks because the airlines block off certain seats for assignment at the airport, and also elite level seating, etc...