It is a commonly believed that the predominant force in partisan asymmetry is population clustering: groups that tilt Democratic are clustered into cities, generating a natural packing effect. A clustering effect certainly exists. However, as of 2012-2014, this effect has become secondary to gerrymandering in a handful of states.
Population clustering and partisan actions are not mutually exclusive. In fact, partisan gerrymandering relies on the fact that voters are not distributed perfectly uniformly. Using this fact, redistricters lasso voters into districts to suit political ends. For this reason, it is easy to mix up the two processes.