It is a bit lower in volume than the other two tracks, but it's still the same volume as Harmonix releases (or just a little bit louder than some of the more recent ones).
So while it is a little lower than the previous two released Ziltoid tracks, the team has decided to leave it as it is. So this is the version that will be released
For what it's worth, I find that slightly lower overall track volume works better for mixes, particularly when RB plays with the track volume even more depending on what instrument you're playing. It also lets folks with high quality equipment use that to pump up the volume instead of possibly compressing the sample and ruining the quality.
Not talking about anything specific here, more a general experience.