The girl has probably said or done something to indicate to the speaker that she didn't grow up in a city. She may have grown up in the country, or in a village or small town, and she isn't accustomed to how things are done in a large city. It could also mean that she doesn't like city life. For example, it's possible to say 'Even though I've lived my whole life in London, I'm not really a city girl - I'm much happier in the countryside.'