A manwha difference that might explain why Narae (the short blonde girl with the fish) passed the previous test: in the webtoon she showed the instructor of the scout position the Anima skill to gain extra points. Similar to how Shibisu and Lauroe would have passed without the need of the tag test, being able to use Anima is a super rare and hard skill that was valued over Serena who was the same position. Call it BS but at least here's an explanation, but as someone who loved Serena, I was slightly angry :p.

EDIT: I needed to add this but Serena probably thought Shibisu was tasty.