In an inertial dyno RAW power (power observed at rollers) is basically the same as the Wheel power (power that is available at wheels), with the only exception of the power lost at bearings, which can be ignored.
But in a braked dyno using air-cooled eddy current brakes, part of the power is used in the air-pump work in the brake rotors, specially for roller/brake speeds over 2000 RPM.
Solution:
The power wasted by the brake in the air-pump effect could me measured doing a high speed coast down test, but this is a complicated test due to the speeds and power levels involved.