When describing a professor's academic position, it is common to say that the person "is professor" rather than "is a professor." That is, you might say, "John Doe is professor of chemistry" rather than "John Doe is a professor of chemistry."

  I'm curious, why is that? We don't say, "Sarah is doctor," we say "Sarah is a doctor." We don't say, "Edward is accountant," we say "Edward is an accountant." Is the idea that academic titles are more formal, so we drop the article much as we would when describing a formal title (as in, "John G. Roberts is Chief Justice of the United States.")? Either way, it always sounds odd to my ear.