Just wanted to share the viewpoint of a neurobiologist, Michael Gazzaniga, who has researched this phenomenon (from here):
“The left hemisphere,” says Gazzaniga, “is a chatty know-it-all, working behind the scenes to piece together a stream of sensory impressions into something that makes sense. The right brain is focused on the big picture, and does not have the same compulsion to figure out the ‘why’ behind events.”
In addition to pinpointing the left brain as the storyteller, Gazzaniga and his colleagues found that when the left brain didn’t know something, it went ahead and filled in the gaps. “The left brain recognizes patterns and pieces data together in ways that fit a pattern,” notes Gazzaniga.
Gazzaniga describes one experiment in which a split-brain patient was shown two pictures. The left brain saw a chicken claw, and the right brain saw a snow scene. Next the patient was asked to select a picture of an item using his left hand (controlled by the right side of the brain). The patient selected a shovel. When asked why, the verbal left brain launched into a detailed explanation of how a chicken claw is part of a chicken, and that a shovel is needed to clean out a chicken coop.
Besides making zero reference to the snow scene, the patient’s left brain was never at a loss as to “why” it had done something. It never questioned its chicken-coop story. “The left brain,” comments Gazzaniga in his book, “observing the left hand’s response without the knowledge of why it has picked that item, has to explain it. It will not say, ‘I don’t know’ ”