Because she is a teenage kid! The author was not good at introducing it, but scenes with her younger self involved are part of a flashback of how she met the chairman (Taeha + Taehee 's father). Her parents were trying to form a merger between their family and the Beom family by marrying their offspring together.
I'm honestly confused about the timeline. I feel like the entire story so far has been a narration of a past tale, with narrator Haesoo (speaking in 3rd person in between the panels) being part of the true timeline of the future. So, rather than a flashback, I'd say the panels in which Taeha and the girl meet is like a parallel timeline happening in the past, since the maim story already takes place in the past. Am I making sense...
Btw, it was Taeha in those panels and not his father Taejoo,,, right? Let me go double check..

Tf is this art style? What happened?? Why every woman looks like a 13 years old?!?!?! Even the mother looks like a kid...