Four words: Nail on the Head. Get used to hearing "Needs of the Marine Corps" because that's really all that ever matters......demands of the USMC.
I think it has a lot to do with the MOS school's training cycle and open slots for that certain time. The word when I was at TBS was that my Co (C '06) had an entire IOC class to fill, hence we got 60 something infantry slots, whereas the next three companies had a class to split between them, so they got around 20 or so slots each............supply and demand.