I think the reason for INTJs often being perceived as extroverts is that what the world around them gets to see is plain old Te. The aggressiveness and the cynical sense of humor which can go along with it, may seem like extroversion to those more reserved. In addition to that, some INTJs have been known to have a certain tendency to give orders to the folks around them...
The Ni part mostly happens in solitude, so people do not notice much about it.