You wanna know what's weird? When I speak to other people, I stutter. When I read a book out loud, to myself, I don't stutter. Not even close, it's completely effortless. When I read out loud to a group of people... I DON'T stutter, at all. But like Nukefudge already explained, when you're simply reading you're not really communicating so I imagine it's just a different process altogether as far as your brain is concerned.