Ive seen it a few times on different carriers. I agree with the above poster, probably either those who had 'em got UPed or were no-shows, and nobody checked again at the gate to see if they opened up.

Something I always do exactly because of this where they do open. Same goes for any seat taht will be closer to the front, it Never hurts to recheck at the gate all you can be told is Nothing better is available or you might be Surprised and get Lucky.