The illusion of superior-intelligence anchors the Know-It-All's self-esteem. While she might indeed be very smart, what matters to her is to be perceived as the smartest of all. The need to be valued for her intellect often compensates for poor self-esteem and, in some cases, for a lack of knowledge.
To ensure her place at the top of the intellectual pecking order, the Know-It-All tries to control everything she is involved with. That way, if she lacks certain information or knowledge, she has the power to cover it up. The Know-It-All is rarely the most athletically gifted or socially magnetic person in the department. It is the brainpower they boast about that is the source of their pride.