In my opinion the reversion at low speed can sometimes make the o2 reading unreliable. I don’t know that as fact. This can be affected by the length of pipe behind the o2 sensor too. I see lean spikes in the o2 curve at lower rpms that don't coincide with the air fuel ratio as calculated by the airflow and fuel flow meters. There could also be others reasons for that too.
All that said about how unreliable o2s can be I have found them to be very helpful (a real time saver) and for the majority of the time a reliable indicator of the air fuel ratio.