When I was living in Boston I had a roomate who was an assistant professor of anthropology. He was from San José. He explained this stuff to me in a way that made total sense. He said that people are not the same everywhere; they actually differ quite a lot from one another. Some of this difference is individual, some family, some subcultural and some from the larger culture. Some groups are much more open to strangers than others. This doesn't necessarily mean that they are looking to become friends, only that they are not automatically cold with strangers.
One of his favorite things was to point out stuff at parties, such as how people stood together in groups. Most ethnic easterners other than Irish are bunched together practically touching, or in fact often taking hold of one another. Westerners were more likely to space themselves out in kind of a loose semi-circle.
I tend to be conscious of these things, and to interpret them as meaningful but not as simple indicators of a single trait such as "friendliness". This kind of knowledge can also be used deliberately as anyone who has survived as a sales professional knows in his/her bones.
I like to have fun with this stuff in some moods. Once a car salesman was giving me the hearty good ol' boy treatment as we were walking along in the lot. I put my arm around his shoulder and said "I am so happy to find you!" He practically fell over laughing.
ha