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It's kind of an odd thing. I think more information is needed as to why the narrator is doing this. For example:
Unreliable Narrator - The narrator is just inventing a new name that is not factual.
Accidental Misnaming - The narrator is making a mistake (usually with a reason behind it).
Foreshadowing - The narrator's names are accurate because Mary will turn out to have a Split Personality that is not outwardly apparent but the narrator knows which personality is in control at a given moment. (Note: This is just an example. I know nothing about this character.)
In-Series Nickname - While "Mary" and "Marcella" are not ordinarily related as nicknames, one supposes it is possible. If this is the case, why would one name be more correct than the other?
Edited by KD
to clarify, her name in the original game was mary, but it's implied she changed her name, due to the fact that some characters refer to her as marcella.
when a character is called a different name by the narrator
- the Ib fanfiction Mary, Quite contrary
. under normal circumstances, mary is referred to as marcella by the narrator, but whenever confronted by garry, she’s referred to as mary also by the narrator.
Edited by c1beat7per9minute