I am not satisfied with your explanation. I just don't get it. In what cases can local custom decide whether the definite article is used? Do they have a rule unknown to the masses?
People don't, in general, follow "rules" when they speak. They speak and that becomes the custom. There is no rational explanation for it.
People on the west coast of the US tend to call their highways "the 405" with an article. In the east, we don't do that. We would just say "I was on 95."
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