According to the manual:

"CONSTANT POWER SWITCH Set the rear panel slide switch to match the impedance of the speakers used. Use the combined impedance value if you are connecting speakers in series or parallel. Choose the nearest match if the value is not exactly 4 or 8 ohms. Note that no damage will be done if the switch is left in the wrong position. However, you may not benefit from the full capability of the amplifier.
􏰀 Constant power explained The typical amplifier is optimised to produce full power into 4 ohms and therefore considerably less into 8 ohms.. The PA Series features a unique facility that ensures you get full power output into either 4 or 8 ohm speakers. It does this by reconfiguring the power supply for optimum current (4 ohms), or voltage (8 ohms)."

Perhaps this is why. They also have the ability through switches to delegate signal to the individual amps for mains and monitors.