[Retric]

Your assuming the delta is based around just the prior test scores vs this one. aka old 10 new 15 or old 80 vs new 85 is the same improvement. However, statistically there is a tendency to regress toward the mean making simply staying at 80 end up as statistical progress. However, I suspect their using a flawed model that ignores the tendency for school districts to pack high preforming teachers on top of other high preforming teachers. To correct for this you need to look at what happens when someone moves from one district to another.

PS: There is a fair amount of momentum in many subjects so teachers can impact not just this years test results, but next years as well. In the end it's really difficult to come up with a high quality model and my guess is they simply did not bother.