This is what it is.

There's a rogue seat map for the 757 flights which shows seats 2AC seats that don't exit. That's why the system doesn't allow you to select them. The actual seating on the plane will start 3AC.

All the 757 operated flights I have been on show this erroneous seat map, although usually it switches from the proper seat map, to the incorrect one at about T-72hrs. I'm sure there's a reason for this somewhere.