I am assuming that the differences you are referring to cannot be attributed to differences in teaching style or presentation of curriculum. I am assuming that you are referring to individual differences in children who are in the same class, receiving in the same instruction from the same teacher, so that the differences in reading outcomes are internal ones inherent within the child and their own cognitive processes, and not due to external factors. This is an important distinction to make before answering this question.
Individual differences in achievement in Reading are many of the same factors that affect comprehension and cognition in learning any other subject. Particularly in young students, their attention spans may contribute to differences in achievement. Young students have short attention spans. For some students, this time is considerably shorter than for others. An inability to concentrate may just be a slight lag in development, and may account for one ...
Discussion of possible causes for differences in rates of learning Reading between children .