I think that it is a fluctuation which indicates it is alive. Remember that the O2 sensor is like a small generator with a one volt potential. The bias voltage from the controller is around 0.45v as the sensor creats current flow this is pulled toward the reading. I think the controller sees this as a randow change in voltage. It quickly heads to the potential generated by the mixture being tested and the controller reacts.
Bosch Master, ASE Master, L1
33 years MB technician