Exaggerated? Statistically, there is no explanation other than people are mistaken or flat-out lying. Asking folks to self report IQ is a bit like asking men how many women they have slept with: some sort of coefficient must be applied if the results are to reflect reality.
(Obviously, there are those too, like myself, who don't believe the various IQ tests and scales are a meaningful measure of intelligence, but that isn't the subject of the poll.)