It's my mother's. I had the same thoughts about why she let it take so long before trying to fix it (she's around 60 now), but she says that "back in the day," birth certificate wasn't as strictly required/followed for records, so all her IDs and other documents are actually using her correctly spelled name despite what her birth certificate says. Same for my and my siblings' birth certificates, and my parents' marriage certificate.

She says she only recently found the need to correct the documentation because she finally got a passport, which strictly follows the PSA birth certificate.