What Causes The Lead And Lag Seen In Standard Accommodative Stimulus/Response Curves?

Lead & Lag Accomodation These two graphs show computed refractions for a model with primary SA as it accommodates; the familiar accommodative stimulus/response curves. This model achieves paraxial focus for all target vergences within the accommodative range. The change in sign of SA as the eye accommodates can cause an apparent lead and lag of accommodation that varies with pupil size (small pupil on left and large pupil on right), which is most exaggerated for refractions that focus more marginal regions of the pupil.
(http://onlinelibrary.wiley.com/doi/10.1111/opo.12047/full)