Why does the pump on a pumped accumulator run on after only using a small amount of water?

An accumulator tank stores water at pressure. When an outlet is opened and water leaves the tank, the pressure in the tank and system drops.

When only a small amount of water is used, the pressure still decreases in the vessel, so the pump activates to re-pressurise. When the pump is refilling the vessel and system, it needs to force water into the tank while pushing against the pressure generated by the water already stored in the vessel. This means that the pump cannot refill at 12L/min so it may take longer to refill the tank.