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.