I'd imagine that some of them where a large percent of short questions are low-quality, but that there are others (like Unix) where short questions aren't necessarily low-quality
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No, there is no fundamental difference between sites which makes short questions more or less appropriate. The How-to-ask page is the same across the network.
It's possible that the Unix community may be more flexible on these rules than others (Skeptics comes to mind as a site which has specific requirements for new posts), but great questions have the same requirements everywhere.