Because the output voltages from each phase of the three-phase AC power supply are different.
During the tracking operation, the master unit of PCZ1000A controls the slave units via serial communication to instruct the same load current to flow.
However, the input voltages are individually measured by each unit.
In constant-resistance (CR) mode or constant-power (CP) mode, if the input voltages for each unit are different, the same resistance or power values set by the master unit won’t be applied to the slave units.
See an example below for CP mode:
The master unit setting: 900 W in CP mode
If the input voltage to the master unit would be 100.0 V, the load current is 9.0 A.
If the input voltage to one of the slave unit would be 97.0 V, and the load current instructed by the master unit is 9.0 A, the power consumed by the slave unit is: 97.0 x 9.0 = 873 W.

If you want the master and slave units to consume the same power regardless of the input voltage, stop the tracking operation and control each unit independently.