The SIP is driven by the AMOR (Annual Military Occupation Review). There is crystal balling that goes on with that out to about 5 years, due to the requirement to try to figure out how many to bring in for ROTP/UTPNCM so they reach OFP at the right time to meet the forecast vacancies and then the short(er) lead time intake programs (DEO, SCP, CFR, OT etc) are fine tuned yearly to fill in the shortfalls or overages that result from that.
For the MP Branch, the AMOR takes place in the late October time frame, I suspect the rest are done at about the same time in order to set the numbers for the next FY. When the selection program opening messages are cut in the August/September time frame they don't know how many vacancies there will be exactly, just if there is a need for a specific program.