In theory, each round of the postseason in any sport is supposed to be what separates out the weak teams from the strong. In theory, it's hard for a worse team to win four games against a better one, even if the margin between them is thin. It doesn't always happen that way, of course, but that's how it's supposed to work.
Consequently, as you advance through the playoffs the series are supposed to be closer; teams that win — even handily — in the first round shouldn't have quite so easy of a time in the second, and so on.