Well, it wasn't Greta Garbo! She cleared things up in a 1955 interview. https://www.smithsonianmag.com/smar...s quip attributed,1955 piece in LIFE magazine.
The question is, Is that just a line written in some movies? When she was actually interviewed, didn't she say I want to be left alone?
The quotes conflict somewhat. The narrator in that clip I attached claimed that Garbo told a friend "I never said I want to be left alone, I said I want to be let alone. There is all the difference." The article Hoot linked to stated So this reference omitted the word "left," most likely to make it fit the famous quote. And it was in an interview, not a conversation with a friend. She should have said "I want to not be misunderstood."