The brake horsepower equation is BHP= (QxHxSG)/(3960xeff) where Q=flow rate, H=head, SG=fluid specific gravity, and eff=pump efficiency. The thing to keep in mind is that a centrifugal pump does its thing by adding kinetic energy to the fluid. The over all result of that effort is dependent on how much energy the fluid has all ready, which is dependent on head. You have noted the relationship between flow and BHP, but if you look at the equation, it becomes apparent that you can? do that. You have to look at BHP in terms of flow and head and efficiency for a constant impeller geometry, speed , and specific gravity. Flow, head, and efficiency all affect power. It is true that as flow goes up the BHP goes up as well, but at the same time the head is coming down, and the efficiency goes up until it hit the best efficiency point, and then it comes back down over the flow range of the pump. To see what the pump is doing requires plotting BHP, eff, and H against flow.
So lets look at it, based on the equation, if flow increases, BHP should increase, Ok, that is the point you made. Also if head increases BHP increases. If efficiency increases, BHP decreases, makes sense. But if you only look at head and BHP versus flow you could come to the conclusion that as head decreases, BHP increases, but that is only because the flow increases faster than the head decreases.
I have probably just confused the issue further....