For the generalized plane strain section, the formulation implemented in ABAQUS assumes the material is bounded by two rigid planes in the 3rd direction.
If the two planes are parallel to each other then the strain component in the 3rd (out of model plane) direction is constant.
However, the ABAQUS formulation allows relative rotations (with respect to the reference point of the part instance) between the two planes. In this case, the current thickness of the section depends on the coordinates (x,y) and so does the strain component in the 3rd direction, which is computed as:
strain33=ln(current_thickness/initial_thickness)