To me, the chance of the Astros, Yanks, Dodgers & Braves making the WS were way higher than the other 6 teams combined. One could argue the Nats were a top 4 team going into the playoffs based on their second half record and strong rotation.
The chance of an underdog winning 3 series vs better teams is very remote in MLB.
In the NFL, it's a one game, single elimination playoff system, so slight underdogs win much more often, and moderate underdogs win more than baseball. Still, in football, the best team wins very often. It's designed that way. That's why the season only has to be one-tenth of the MLB season to weed out the best from the worst.