It actually is pretty common (look how much better Mass Effect 3 is with The Citadel & Leviathan DLC, or Dragon Age Inquisition with the Trespasser DLC, or Deus Ex: Human Revolution with The Missing Link, for example) but it isn't a for any nefarious reason.

For the most part, it's just the plain fact that DLC is made at the end of the cycle, after all the creative people working on it have had the most time to get the feel for the worlds they create. And this experience translates in to better worldbuilding.