9-12 applications are not given as much scrutiny as 12-15 are.
I guess that those who are applying from 12-15 are generally quite senior SAC's whom, you have to wonder why they are not already JNCOs, whereas your 9-12's are generally more junior who will have the potential to have reached Cpl by the time their 12 yrs is up, and would then automatically be offered 22 yrs.
All ACOS are doing is signing up those who are showing potential at the earlier stage, and then filling up any gaps with the plodders who had their chance to show potential many years ago.