She was 5 years old when left on jakku, accents form at an extremely early age. I doubt you've talked to every person on jakku to see if they have a similar accent
Think about Star Wars characters -- especially the original trilogy. Who has upper-class British accents? Imperials and Obi Wan. Makes sense -- snooty people and learned people.
Jakku is a rough and tumble backwater. Makes no sense. People on Tatooine didn't speak like English royalty (again, Obi Wan being the exception).
As for fixed accents forming early, that's bullshyt. Kids unlearn accents if they move somewhere at 5.