Icelandair has made a good business of operating Boeing 757s on routes from Iceland to the US with Seattle being a regular destination for years. There was a time when the 757 was a cutting-edge jet but these days they are getting up in years. Similarly, the RB211-535E4 was once considered very advanced. Now, it is dated. It turns out that one of the engines on this flight is in a little better shape than the other. As they cycled the power on the approach, the starboard engine was noticeably smokier. Not a big deal but I guess it has been on the wing a little longer than its partner.