I hate to flog a dead horse, but in my case at least if I read without music I'm subject to all sorts of irregular noise (cars, doors, talking, other people's music, etc) that is distracting. Listening to music I know well isn't distracting because my ears are familiar with the sounds and so they don't tell my brain to pay attention. Or something like that; I have no idea about the scientific aspects of this.
I wish I knew more about how the brain works. Of course, I'll probably be taking a cognitive neuroscience course in the next couple years, so there we go.