An excerpt from Elena Kagan's opinion in Caraco Pharmaceutical Laboratories, Ltd. v. Novo Nordisk A/S, which was decided today (emphasis ours):
Truth be told, the answer to the general question “What does ‘not an’ mean?” is “It depends”: The meaning of the phrase turns on its context. See Johnson v. United States, 559 U. S. ___, ___ (2010) (slip op., at 5) (“Ultimately, context determines meaning”). “Not an” sometimes means “not any,” in the way Novo claims. If your spouse tells you he is late because he “did not take a cab,” you will infer that he took no cab at all (but took the bus instead). If your child admits that she “did not read a book all summer,” you will surmise that she did not read any book (but went to the movies a lot). And if a sports-fan friend bemoans that “the New York Mets do not have a chance of winning the World Series,” you will gather that the team has no chance whatsoever (because they have no hitting).
Kagan is a New York–born Mets fan, but she apparently isn't feeling very confident, despite the team's 7-3 start.