Widebody jets coming into SEA are hard to predict.  If possible, all arriving traffic is sent to the outer runway to allow departures to proceed from the inner runway with little disruption.  However, if there is a lot of arriving traffic, the wake turbulence requirements for spacing behind a heavy jet can slow the arrivals flow.  In this case, sending the jets the inner runway is more efficient.  You never know what it will be until the plane is lined up on approach and you can see whether it is offset from the normal paths or not.

I wasn’t terribly bothered by this American Airlines 777-200ER when it came in as it is a daily arrival from London, and I have shot it on previous occasions.  However, since I was in a location almost on the centerline of its approach, I decided to go for more of a head on shot and then an underside shot.  If this was something I hadn’t shot before, I would be aiming to get the side of the plane in shot to show whatever it was but, in this case, no harm in playing around with different angles.