I wasn’t aware the rule was based on phonetics; I was one of the many under the impression that the actual lettering that is the defining article (much like OP). I think it’s due to being monolingual. English being the only language I know… a lot of the languages rules are known subconsciously.

IE: I knew it’s ‘a one’; rather than ‘an one’… But couldn’t have told you why, other than it doesn’t sound right.