A more likely explanation is that when you have short series (3-game, 5-game), there are going to be upsets. Good hitting and pitching teams have slumps. I'm sure if one looked back over the regular season, we'd find stretches where the Braves, Orioles, and Dodgers didn't pitch or hit well (I fondly remember the Giants sweeping the Dodgers back in June, outscoring them 29-8 over 3 games). During the regular season, those slumps tend to even out (Dodgers made the playoffs; the Giants didn't). In the post season, however, they don't.
Notably, even a 7-game series isn't long enough to guarantee that the best team will win. As statistician Leonard Mlodino has noted, if one team is expected beat the other 55 percent of the time, the weaker team will still win a 7-game series about 4 times out of 10. Or, if one team is expected to beat another team, on average, 2/3 of the time, the other team will still win a 7-game series about 20 percent of the time. In fact, in the latter case, teams would have to play a series consisting of at least 23 games to guarantee that the best team will win. And in the case of one team having "only" a 55–45 edge, the shortest series that would guarantee the best team won the series would be 269 games.
That's a lot of games.