An hour and a one: The truth about a and an

Why is it "an hour" and "a one" when hour begins with a constanant and one begins with a vowel? The truth is the rule states that the word an should be used before a word with a vowel sound not a vowel. Therefore the word hour with it's silent h gets an "An" and the word one with it's unscripted "w" gets an "a". This common in romantic languages but unlike romantic languages English has silent and unscripted letters.