Within the model, riparian strips only remove P - they have no effect on N. The P runoff is a calibrated risk model that predicts loss to second order streams. A lot of this P is attached to fine particles that do not readily settle out.
Riparian / grass filter strips capture runoff. On blocks with low rainfall, modelled runoff is low or non-existent and so the riparian strip will not impact on the modelled P-loss.