As the weather gets colder, defense becomes even more important. Not just any defense, but real hard hitting defenses. As some offenses start to play more conservatively, defenses will be worn down even more. Defenses that succeeded early in the season by gambling on interceptions will probably regress as the end of the season approaches.
These two teams have conservative, mediocre at best offenses. They also happen to have two of the best defenses this season. Normally two defenses that hold opponents to such low averages (15.3 OPPG for SF, 19.9 OPPG for WAS) would end up with a line below 35. However, because they are not big name teams, people don't associate them with dominant defenses like Pittsburgh and Baltimore. And that is where the value lies.