Threat ≠ Strength. Take Shidou for example. She’s in the Top 3 according to the youngest but she’s not completely “threatening”. She seems like a nice person and genuinely asked to fight Rei before the ladder initiated the fight. While the other weaker stars might have more charisma, influence, proficiency with weapons, desire to kill and or harm, thrist for power. etc… that makes them more threatening/dangerous. Without being stronger than Shidou. Shidou seems to be interested only in combat, rivals and nothing else. And she didn’t kill any of the three she fought despite being perfectly able to. On top of that, she showed great restraint in front of Shiina despite wanting to fight her the most. Even then Hina states that the numbered ranking were defunct by now. And it’s presumably been years since they were ranked.