Relative volume fools people all the time, even trained professionals!
Have you ever inserted a plug-in and felt the mix sounded better, then actually matched the level with the plug inserted and without it, only to realize it actually doesn't sound much different, or even sounds worse with the new plug? I have!
It's the same with a CD. The first thing the band or whoever is going to do is take the CD and put it in their stereo. Then they'll pop it out and listen to something they're familiar with (which, if released in the last 10 years will be LOUD) Then, pop the new CD in and notice that it's 'small, not exciting, doesn't sound as good as the other one.' It can be 100% a volume difference, which if they corrected by turning the volume knob, they would hear, but they don't want to assume people know how to operate their volume knobs!
I remember years ago, we had a little punk rock label, and didn't know anything about mastering. The first CD we released was easily 12db below commercial CDs of the time. And a few people commented on it, etc. But college radio still played it, and no one ever returned it. That was in the days of CD tray machine, where you'd put in 5 CDs and hit 'random'. That really made the level difference obvious!
I hope that in the future, we see more consumer gear able to adjust a track so the the RMS (or perceived level, more importantly) is consistent from song to song.