In order to explain vertical and east-west type I asymmetries uncovered in a companion paper and in references therein, we introduce the notion that the generating large-scale gradient drift waves break down first in elongated isolated enhancement and depletion regions (blobs and holes) that follow an asymmetric nonlinear evolution. The nonlinear evolution is accompanied by a rotation of the electric field inside the structures, be they blobs or holes. It is shown here that holes have a tendency to rotate more than blobs but that as a result the electric field inside holes is also weaker. These two features (contrasting rotations and contrasting field strengths) explain the predominant sign of the up-down and east-west asymmetries during weakly to moderately driven conditions. They also explain how the asymmetry is observed to disappear and even to reverse its sign under more strongly driven conditions.