I could be wrong but your mind wandering off on a tangent while you're reading might be a sign of creativity. Creativity could be when you find relationships between two things that might not have been initially connected in your head. Your mind wandering could be your brain literally making new connections between the new material you're reading and existing material in your brain. Also, I think comparing and contrasting information is a way of learning. I'm basing this from a book I read, A Mind for Numbers, and they have a chapter that explains how the brain works and thus how people learn.