The question has established that the P-B portion of the variable resistor is greater than the internal resistance of the power supply, indicating that any further reduction in the resistance of the circuit (until it is reduced to the internal resistance of the power supply) falls within the range of values of the domain in which the output power of the power supply increases.
When S2 is closed, the circuit becomes a variable resistor A-P part of the resistance of the bulb in parallel, and then connected in series with the P-B part of the variable resistor, although than the original total resistance has been reduced, but the total resistance is still greater than the resistance of the variable resistor P-B, that is, greater than the power supply's internal resistance. At this time, the resistance is reduced, still belongs to the power supply output power increase in the domain value range of resistance change, so the power supply output power is still larger.