It was just a misunderstanding on Jaemin's part. Seungpyo had already realised and admitted that he liked Jaemin, even though he was a guy. Seungpyo truly felt that way about Jaemin. However, Jaemin, who was still in denial, thought Seungpyo was just messing around with him because they always had that bet things (ᵕ—ᴗ—)
Yepp, if I remember correctly, after the exam results come out, Seungpyo was in second place (I've forgotten some details.) However, it was certain that Seungpyo's results weren't as good as Jaemin thought. So Jaemin asked Seungpyo about it, and Seungpyo explained that he didn't want to make those bets anymore because they were already a couple. (Again, this is if I remember correctly, because I read the novel last year.) •ᴗ•

Omg I'm in NEED for spoilers