bum has really changed he has gotten more confident imo

Yeah, i think that's why he "won", Bum had originally acted like that because he was locked away with his abusive, rapist uncle and now Sangwoo sees he's taking up a big enough space in Bum that he's changed. If you look back, Sangwoo talks about this before the amusement park, about how Bum's uncle has "ruined everything" and how Bum has lived a sheltered life.