Ummm... Actually very often the opposite is true.
I've interviewed a number of people who were incredibly confident in their language, voice and body language who clearly had no idea what they were talking about.
Similarly, I've interviewed people who clearly knew way more about the subject matter of the job than I did but who were incredibly nervous during the interview either because they were intimidated by the idea of working outside of academia, real/preceived language limitations, or because they were just incredibly shy and awkward.
I've generally tried to look for people who fit the sweet spot in-between: confident and sociable enough to be reasonably comfortable during the interview and humble enough to not be brash. The more "customer/client" facing the position, the more I care about the confident and sociable part.
