I wouldn't worry about it. I applaud any reading that is self-motivated -- one of my kids is dyslexic so I have heightened appreciation for any book that she picks up independently. A one year difference between what your child can read with you and what the child reads independently doesn't seem out-of-whack. Reading is a multi-level process. Decoding and comprehension don't always proceed at the same rate. Reading at a lower level is good practice and solidifies comprehension skills.