"severity" and "subsequent chance" play different weight while the referee makes the decision during the devleopment of the game.
Severity implies the nature of the foul would be the serious one which might be exceed the level of technical foul. As I believe it will harm the opposite play in which lay off shall take immediately in no more consideration.
Subsequent chance shall not only be limited to any score opportunity but also applies to the opportunity to facilitate ball playing by opposite team. So that's why these two factors play different weight during the game.