So if you knew a few, and they did act that way, would you presume any who didn't to be faking?
There are a freaking billion of them. My guess is that they are a diverse group of people with different personalities, different values, etc.
But why should it be a surprise when someone acts roughly in line with a stereotype that applies to them? Stereotypes, while they can have negative effects, often arise from observed behavior (especially the benign ones).
(Last night I watched Breaking Bad with a few friends, and we drank wine, craft beers and homemade iced tea, while eating from a plate of fancy salami, blackberries and Japanese pears. You might be able to use your knowledge of stereotypes to guess my race.)