Even though I've lived all my life in Sydney's southern suburbs, I've had many people over the years wonder where my accent is from. But I'm Australian!
What I think has happened is that the same things that originally engendered the early versions of the English 'Received Pronunciation' at the likes of Oxford happened to me: namely, the influence of reading a lot. As a child, I was a voracious reader. When I moved out of home a few years ago, even my mother did not fail to be surprised at just how many books I own. And I've read almost all of them.
This was borne out a few years ago when I heard a black American preacher who had spent a lot of time in university. His accent was very unusual: to a lot of people it sounded a little British, but not quite, yet we knew he was from the US and could here a lot of New York in there, too. It took a friend of mine who is even more accent-aware than I am to point out that it's because he has so much education. It was an educated accent.
Wade.