One possible possibility could be your gf's seat was originally beside a DM / baby / elder / etc. And that party wish to be seated together with another pax, thus needing her seat.

Since your gf was travelling as a single pax, it was easier to move her around. At the same time, CX does not want to anger anyone by reseating her to a random Y seat. So since there was an availability in PEY - they chose to just op up her.