I thought yumi kanchan and natsuo are friends during highschool. So why is kanchan 27 whil...

maybe, the stories did not happen in parallel, but at other times..

I think it's cause Natsuo and Hiro's story happens first. I'm pretty sure Yumi and Mayama were 24 when they got together, i swear they were also younger. So it must have been several years since Kan-chan broke up with Yumi.