Have you noticed that some southern people have strong accents and others have a slight accent?

It's about how one is raised. Here in Australia most people think that the heavier accents only occur in inland, rural areas, but there are alot of people who live in slums or low socio-economic areas that have just as heavy accents. The reason why Australians who live on the coast, and in the cities sound more like a British accent, is just because that's what our accent sounds like when you speak clearly and properly.