The simplest answer is that she wasn't born in St. Albans. She might have not known where she was born, but that was the place she remembered growing up.
Or, she might not have known exactly how old she was. She might not have provided the census information at all and the person who did guessed. Or she lied about her age.
Or, she might not have been an O'Brien when she was born -- her mother might have (re)married after her birth and she took her step-father's name. Or she had been married. Or she just decided to adopt another name... Changed her first name. Used her middle name...
You have to widen your search -- by date and place and name. Also try other index sites such as Freebmd -- it's not unknown for indexers to make a mistake, but less likely that two totally independent indexes will make the same mistake.
You should also try looking for her in earlier censuses as a further clue.
However, be prepared for a long slog -- I have an ancestor that I have been looking for in the birth indices for over 15 years: Does the available evidence support these assumptions?