The question of who stays healthy over the regular season is an entirely different subject, though an interesting one.
My point, in case it's not obvious, is that any team can have a bad day. Your Pats, for instance, lost at home to the mediocre Broncos during the regular season. Does that mean the Broncos are a better team?
I understand your point - BUT - there's only 16 games in the season. Would you have the Pats and Chargers play a 3 game series to eliminate the "luck factor"? That's not practical. So, my point about the regular season is that, just as luck is a factor in "only" one playoff game, so it is in every game during the season. Look at how KC got into the playoffs - on a missed FG by the Bengals. The Bengals make the FG and it changes the wild card matchups. Maybe Cincy beats the Pats. Maybe Denver gets in and SD beats them. There are bounces in every game and given how tight the NFL is in terms of good teams, it's right that there's one game.