I’d say this is mostly an experience thing, but note Shuu did say Seya was a 知らない人. Also note the way the characters talk to each other - Seya is very polite to Shuu because they don’t know each other, but when he gives the notebook back to Maho they’re super casual becaus they’re classmates and apparently talk. Maho is even like せやじゃん. (I don’t have the book in front of me anymore and don’t remember how his name is written)
A lot of times in Japanese you have to use these cultural language things because they imply a lot.