I think it was a glitch that let you book it in the first place. Normally, you have no choice and definitely shouldn't be allowed to book an exit row. Although, I once was able to reserve 3 seats next to one another on a saver but the other two were minors which I assumed was the reason. I've only booked saver fares a couple times but it seems like seat selection thing is a bit inconsistent. The whole seat selecting/reserving and getting moved around is always a bit of a fluid situation with AS. I always check my flights periodically and especially in the days before the flight. I recently got bounced out of my premium class seat on two legs of a flight twice in the 24 hours leading up to departure. Ended up in a middle seat in the back of the plane. Glad I checked!!