Bah, I was too slow. I already replied in the other thread ya went and hijacked. The easy explanation is "that's how centrifugal pumps work". The more technical answer is that a pump creates kinetic and potential energy. The kinetic is fluid motion while the potential is pressure * flow. When you monitor pressure and flow and do the calculations, you'll find that power is more sensitive to velocity than pressure. Translation: More flow at lower pressure equals more power than low flow at higher pressure.
Efficiency is another interesting aspect, but not one I wish to toss into the mix here.
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