I also used to hear something like what Syanjali has mentioned. Such kind of inferences must have some links to historical observations. Jhapalis being cleverer is understandable because, compared with the other folks who stayed in the hills, they were cleverer in the first place to realize the the benefits of migration. Also intermixing with the people of different decents and particularly an interaction with a relatively forward and educated community in Darjeeling must have helped them to be "smarter" in some sense. And, it'd be just a matter of few incidents of working together with ordinary hill folks that smartness would manifest itself in the form of cunningness, as is common in cases of smartness devoid of a corresponding ethical counterpart.

In general, I used to hear a lot about Purbeli and Paschhimeli, the former sometimes being considered  smarter than the latter. Of course, such generalizations were based on personal observations and do not appear to hold any more. Environment, hardships of survival and many other factors would probably contribute to a particular group being cleverer than the rest, and in the present context, it can be true that the (migrated) people in the plains generally outsmart ordinary people from villages in the hills.