As it says in the blog, these are lists of Phase 3 worlds. SE may plan to increase the number of new worlds even during Phase 3. I don't know their capacity, but for Phase 4 and certainly Launch, I think it's likely they'll need to add more new worlds.

For Legacy worlds, the number of characters is already known. There are active characters and a lot more that have been dormant for years. SE probably has analyzed all this to predict how many will reactivate and which region they'll go to. Also included should be enough capacity for new characters on legacy worlds.

This would be a lot of variables to analyze and hopefully they will predict correctly. If not, maybe they could steer some transfers by marking worlds "full" (leaving space for new characters to join), or even adding new a new Legacy world if needed. Of course, this is just my guessing. SE has the plan and I don't know what it is.