I think more than anything, it shows how much grammar has changed over centuries. Some things change and some don't and so the English language has become very modular as a result. Bill Bryson explains the evolution of the language in his book, 'English: The Mother Tongue and how it got that way', which I recommend to anyone interested in the evolution of the language. It explains why this rule is used in some places depending on the culture as the language has shifted according to region. I think it's fascinating.
Years ago, I used to write for a gaming website which is no longer around, and we got into a conversation on this very subject. He had used An in a sentence, which to my ears didn't sound right in the context of the sentence. I don't remember the exact details, but I do think its use depends on the word it follows. Too many times these days, I see people either use An or A too often without regard for the word, and the result is often jarring. Personally, I'll say An Historian, as 'A Historian' kind of makes you pause while saying it. That link is a good explanation into why some words like Historian sound good to the ear when coupled by An whole words like hotel don't. Generally, I tend to trust my ear. If it sounds right, it probably is.