It's quite complicated to be honest. Mostly because pumps may use the same fan headers (on the motherboard) as classic fans. We prefer to group items by unit. If you dislike the way and logic that the Computer / Sensor page shows the sensor readings, you can use the OSD Panel or SensorPanel features to have the sensor readings listed and displayed exactly the way you want.
As for "what's the difference between a Flow Sensor and Water Flow ?
They use the same sensor but different units ?", they are measured by different methods and yes, they also use different measurement unit.