Instead of a compromise mid point flow … they have a high and low stage as the vane assembly changes position within the axis of symmetry.
Otherwise how would you explain the rapid increase in pressure when I first punch it rather than when it reaches max RPM on that gear.
They are also feeding piston jets in more and more cases …
Yes, they are typically "two step", but didn't want to get into all that. Basic point is say for example at idle, a variable flow PD pump will be putting out less flow than a non-variable pump at idle, or even less flow at higher RPM depending on how the pump is designed and computer controlled.
The whole goal for the engineer of a variable flow pump is to reduce the pumping loss (HP) as much as possible to gain fuel economy and still provide adequate lubrication to the engine and still make it last it's design life goal.