It seems to be based on demand, availability, and projected occupancy at the location. I've seen some timeshares with point availability 11-12 months in advance and then it go away, and sometimes it opens up as it gets closer. Haven't figured out any logic to it, except that if you want to get a high demand location you have to try to book early and keep checking.

I would also call reservations, because the timeshare point redemption doesn't always show up on the website.