The game starts out fairly hard but then drops off in difficulty once you start leveling. It's almost as if you outlevel the content.

I think maybe they had a lot more content in act 3 at one point, cut a lot of it out, this leada to compression of the leveling timeline without rebalancing the previous content. Thus, you level to quickly. It's supposed to be a longer road to 12, but 15% of the content was cut and leveling rate escalated as a result.

Pure speculation of course.