Issues regarding the frequency conversion effects of frequency converters when used with positive displacement pumps and centrifugal pumps

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As is well known, the main function of an inverter is to change the frequency of power. As for pumps, my understanding is as follows: 1. The purpose of using an inverter with a centrifugal pump is to reduce the pump’s speed without requiring a high flow rate, thereby reducing the pump’s power consumption and achieving energy savings； 2. The purpose of using an inverter with a positive displacement pump is solely to precisely control the material flow rate ； Below is a set of data I have collected to support my understanding above; I’m not sure if it’s correct: For the screw pump, the rated power is 2.2 KW and the rated current is 3.9 A. When the frequency is adjusted to 10 Hz, the phase current measured is 3.0 A ； 3.4A at 20HZ ； 3.7A at 40HZ ； 3.87A at 50HZ ； Centrifugal pump: Rated power of 30KW, rated current of 56A; when the frequency is adjusted to 42HZ, the phase current measured is 25.6A ； I wonder if anyone has noticed this issue: when the speed of a screw pump is reduced, its current decreases only slightly, whereas when the speed of a centrifugal pump is reduced, its current drops sharply! ! ! ! ! ! ! ! ! I hope everyone can explain the reason to me