They are right at the back of the plane, where it starts to get narrower. As a result there is not quite enough room for three seats in the outside row, so only two seats are there. The advantage of this is a little bit of extra room by the side of the window seat, and not having to ask a stranger to move everytime you want to stretch your legs or visit the loo! Ideal for couples, and we also ask nicely for those seats when we fly on 747s
Regards
Rob