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.