It’s ether his age that he’s lying about or where he’s from
age for sure and possibly about where he lives too, but idk. my guess is that he's 1 year younger than Geyol, so 26. Someone else in the comments said the format is year/month/day. Yeseung's ID said: 9Y0329 and Gyeol's is 9X0731. Y comes right after X in the alphabet and therefore, if Gyeol is 27, then Yeseung would be 26.
Well it was cringe but it made me feel like reading those manhwas in like 2020 lol. Personally i prefer the other couple and I wish there was a side story for them or sm:(( And also, the down grade of the art style at the end is mad