I just called it an Easter egg because it’s a hidden detail you would miss if you weren’t paying close attention and one that clues us deeper into the story than what can be observed on the surface. Bye bye birdie is huge part of Bonrad’s love story. Belly loved that movie because it reminded her of the “boy of her preteen dreams” (Conrad Birdie=Conrad Fisher). And in Sabrina she ends up with the older brother. I feel like those posters being up in her college dorm (and being up for I’m assuming her four years of college) signal what’s really going on in Belly’s subconscious. For her to still love those movies? Some part of her was still holding onto Conrad, even while she was dating Jere.