I Knew It: Ayumi being Mercury's host. For the Genre Savvy, the author pushing Chihiro only made it more likely.
Classical Mythology combined with one of Apollo's comments about how she ended up in Kanon because she's the god of music. Of the candidates, who fits the God of Travel most?
Subverted in the case of Tsukiyo; while Classical Mythology would have put Diana, goddess of hunting, poetry, and the moon in her, she instead got Vulcan since Diana was already introduced as the one inside Tenri.
No Export for You: The light novels. The first one had two exclusive capture targets in Tooru Amami and Asami Yoshino, while the second had Shino Akuragawa and Aoba Fuse.
Also, Keima's first capture was Mio, who had a more normal physique and no twintails, and was still living in luxury. The prototype also did not give captured girls Easy Amnesia, so she remained infatuated with Keima, much to his chagrin.