It's really just one of the few theories that I'm thinking of right now.
I'd say it's also possible that the meeting at the shrine may of happened before the biting incident and that was actually Kawashina's brother and Luna was actually turning him down which could of resulted in him calling her a slut out of spite.
It could also just be Atsushi not putting much effort into changing Naoto's appearance, but he definitely looks the same age in all the flashbacks. So I was running with the theory that these flashbacks are taking place from the same year, but it's very likely that Atsushi just never made much effort to distinguish Naoto's age.
I also considered that Luna was closer to 5-7 years older than Naoto and was possibly around the time she would be entering high school.
The other theory I have regarding what may of happened is one that I really hope isn't where this is going, because I'm not really a fan of using it as a plot point in a series.
It's entirely possible that Luna may of been sexually assaulted on the last year she visited and the reason she never went back was because of the trauma from it and the reason she went no contact with Naoto is because she thought he knew and would think badly of her. After the assault her family would of moved away to get away from the place it happened and she would never return to their home town again. This could also explain why Naoto's mother wouldn't tell him why she never came back, to protect him from knowing about the horrible thing that happened. It's also possible that her hair colour is a result from the traumatic event, and isn't actually dyed that colour, aka
Mary Antoinette syndrome.
I
REALLY hope that's not where the story goes from here though.
It's also possible that some other traumatic event may off happened, but I'm really hoping I don't see a sexual violence tag added to this series. Or it could just be some silly and stupid reason, who knows.
I'm still speculating that the home town is in Hokkaido, but this is mostly because of Hiba's surname and (from what i've read anyway) Atsushi is originally from Hokkaido.