If you think of it, really, the NB is least accurate at stoich. Why? Because any voltage ranging from .25 to about .65 could equal stoich. The reason for switching is to keep ecm "in the region" where O2 voltage equals stoich. Moving your switch point outside that range requires greater accuracy in keeping the fuel delivery and O2 feedback closer to each other. The amount of change in AFR vs the amount of change in voltage is what needs to be dealt with. While the ecm is still adjusting fuel lean -> rich -> lean to make voltage swing below -> above -> below switch point, the amount of change in AFR is greater when voltage switches from one direction to the other. Tighten up the loop and allow less variation to reduce the AFR swing and keep voltage swing closer to switch point.