I believe that the SRG requirement is 12 months valid on a position, whether it is first validation or subsequent. A local agreement at LATCC many years ago added the extra 12 months but these could be waived with the agreement of all parties. The usual reason to shorten the waiting period is the time of year. It makes far more sense to have a new OJTI begin instructing during the winter months than pitch them in in May or June just because that is the anniversary of their validation.
We have the biggest training commitment of any unit in the country, so we need the Instructors. Fairness doesn't come in to it.