UIL assignments are different from crew selection. All UIL Assignments are simply the UIL selecting a Chapter for the game assignment because the schools could not even agree to which Chapter to use, much less a single crew. So, in that case, the assigned Chapter will select the officials to work the game.
All of these games, on the other hand, are crew selections made by the schools themselves. They have either used these crews in the past, or have called around, and these crews have been recommended to them.