How pump flow rate is calculated when HeaderedPump:ConstantVolume is implemented?

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I am modelling primary constant and secondary variable system for plant loop.
I have tried to use HeaderedPump:ConstantVolume and expected that the number of pump being operated will vary like below image.

However, actual operation is like red dot line. Every time if there are cooling load, all the pump is being operated.
Does anyone have idea what is causing this problem?