Lets do this comparison. You have states like MD where the kindergarten cutoff date is Sept. So now a Sept 2000 kid in NY would be a 2018 grad. In Maryland he or she is now going to start school next Sept so will be a 2019. Parents hold back and now is a 2020 graduate. Now this MD kid decides to go to a private after doing a 9th grade year in public school and will repeat that 9th grade year at private so is now a 2021 graduate of high school. Happens all the time.