Why can't teens we make in other active households spawn as high school classmates?

I haven't had many problems with the game pulling from the pool of teens (There's so many in terms of premades and the ones I made) I have. Though I noticed there are some that are required that might be game made due to their traits and being on teams as well. Like it one save file I guess I didn't have any teens in the world that was mean so it made one in another file it pulled for the premade Wolfgang since he has the mean trait. The roster seems to vary day to day at times.