IMO, the major reason I find Act 1 and ACt 2 so have such a range of difficulty is because of how free-roam heavy those two acts are.
Act 1 expect you to roam around to get gear and be strong.
Act 2 expect you to roam around to level up nd get stronger.
The moment I got to Act 3, the difficulty felt just right because you barely level up now (Act 1 is like, 1 to 8, Act 2 is 9 to 18, Act 3 it 19-21.
With such a low level range to cover, they manage to make a map that has a forced flow everyone follows to clear the content based on their experience with Act 1 and 2. Because of that, the difficulty went from "Too easy" or "Too hard" to "GODDAMNIT!" nearly every fight.