I was browsing through my Return of the Straight Dope which I bought a while ago in my civic duty to keep this place afloat, and came across a column where Cecil admits to a lacuna in his knowledge, which I am in a position to fill! I’m so excited!
(Okay, so it’s a column about how they used to assign area codes, which is of mere historical relevance ever since the Great Area Code Divisions of the mid-90s, but I’m still excited.)
But he then goes on to add:
I don’t have a cite for it, but the explanation I read long ago in a Canadian magazine that had a proto-Cecil answering column was that area codes with “1” as the middle digit meant that the state or province in question had two or more area codes, while state or provinces that had only one code for the entire area had codes with “0” as the middle digit.
This is consistent, of course, with Cecil’s explanation for why codes with “1” are normally assigned to heavily populated areas. Jurisidictions that are heavily populated will need more than one code. But the code also gives a bit more information: a code with a “1” was a sign that the code would not apply to the entire state/province. If a state or province had a city the size of Boston or Montreal, warranting its own code, the rest of the state or province would get a different code, with a “1”, to indicate that the area code will not apply to the entire state or province.
This explains why D.C. had a code with a “0” - since the same code applied to the entire District, it got a “0”, not a “1”, even though it was heavily populated. The anomaly wasn’t in the code, but in the fact that D.C. has the heaviest population density of any of the jurisdicitions.