Why is the relative power output lower when shooting in HSS mode?

Normal flash willl illuminate the camera sensor fully, as it will sync when the camera sensor is fully open.
HSS will start illumination before the camera sensor opens, and end after the sensor is closed, i.e. it will waste light before and after the actual image is shot.

The second factor to this is the cameras sensor readout speed, that will determine the length of the HSS flash, where some camera manufactors uses a quicker shutter speed than others. This can cause question if one compare two brands light output in HSS, as Profoto devices do not indicate the different HSS durations used.

In general HSS will remove 2-3 stops of light compared to normal flash, dependent on the HSS duration used.