The PCM compares the MAF output to what they calculated for VE table, if the values are not close then PCM assumes something wrong with MAF and switches to VE tables

If people do not muck around with porting MAF or TB then VE tables are only used for startup and fallback conditions.
Once people screw with the airflow path and/or MAF and VE tables then more chance values do not match forcing PCM to switch back/forth to VE tables.