Imagine if you stopped the blades but the craft is still traveling forward... lift is being generated by the air flowing over one side of the rotor just liked a fixed wing aircraft.

At the extremes some full size helicopters can't climb straight up, they have to have forward motion to generate enough lift to climb. This allows use of a smaller motor to bring operating cost down.

In the case DJI drones the battery times are usually quoted with some amount of forward speed.. it generates a longer run time than a hover test.