I would say that the point is that you can't just look at one datapoint, especially if there are other things affecting it.
The most obvious case of this is comparing private vs public schools, where the private schools can be selective and kick out anyone who doesn't perform or they don't like, but the public schools have to accept everyone by law.
Obviously failing anyone who cannot read from getting to 4th grade will greatly improve 8th grade reading scores.
Those failing kids eventually make it to the 8th grade, however, and affect statistics. Still, having lived there and attending one of the better middle and high schools near Vicksburg, I wouldn’t be surprised if they were gaming the system in some way (I hope they aren’t and these gains are real, though).
If a kid achieves a great 8th grade test score at age 18, is that a success or a failure of the system?
What we care about is the level of achievement by a given age. To determine that, we need to be comparing states using standardized tests given to age groups, not grade levels. It is fine to hold students back, if we think that will do them more good than advancing them. But they still need to be tested the same way as their age group if we want to do a meaningful comparison between states.
The most obvious case of this is comparing private vs public schools, where the private schools can be selective and kick out anyone who doesn't perform or they don't like, but the public schools have to accept everyone by law.
Obviously failing anyone who cannot read from getting to 4th grade will greatly improve 8th grade reading scores.