I believe there is a method to the madness with seat assignments. Unaccompanied minors or escorted passengers are in the very rear. Upgrades are in first and premium. Exit rows are often popular, too for leg room.
There's a bit of a bathtub effect where the Super Saver fares and non-revs fill in the area in question. If they didn't have 100%, that's a likely area to have a vacancy. Not much of a mystery and why suspect something? The seats weren't on an MEL.