I'm guessing part of it may have to do with the retirement of the 737 fleet from UA's network. Even if a 737 didn't do the SEA-SFO runs, could still be a result of the way UA is repositioning aircraft network-wide as a result.
And with that in mind, I've noticed that most of the 737's disappear off the schedule as of October 25th. So maybe there is something to my thinking.