Comentário oficial
You can't compare the output from a program which bypasses the channel gain stage to the DJ application which goes through one of the channels.
I have measured the output levels when adjusting the master to compensate, and both rekordbox and iTunes will be at the same volume when the same levels are shown on the master meter. Yes, when using rekordbox I have to increase that level, but that's simply because the signal level coming through on one of the channels is lower, and I have to adjust the master to compensate, whereas the audio level directly through iTunes is higher (but can also be controlled through the volume slider in iTunes).