DVOA caps long plays and, to my knowledge, ER does not. That might be why Seattle rates a little better in Brian Burke's model.
The Seahawks benefit both by their ability to stop long runs--which boosts their run defense ranking all the way to18th--and St. Louis's inability or unwillingness to pass farther than five yards. Over the last two seasons, the Rams have been the only team in the NFL to average fewer than 10 yards per completion. Pat Shurmur is like the anti-Mike Martz.