Why have I got a stronger accent than the people in my family?

I was born in London and not many people in my family have "posh accents" my mom and dad don't.. only my aunt does, and her daughter (my cousin) and for some reason I do? by the way I don't mean "hugh grant or kate winslet posh" I mean more like Kate Beckinsale, Robert Pattinson and Keira Knightly posh" and my twin sister doesn't have a posh accent, hers is just normal, WHY DO I talk more posh than my family?!?! is it just the way I speak or what? I didn't realize how different I sounded to them until I moved to L.A. when people kept mentioning it to me and my mom always calls me her "posh princess"...... WTF? lol.