i may be wrong but Webside Story i think only tracks site that use HotBox. Becuase this is a mainly free web tracking service for small and amateur websites i would take the percentage as being valid in any shape or form for these sites only. I would guess these sites invest less in SEO, and may not have as much content, when content is favoured by SE's at present.
Id they had really good content they would probably invest in a better form of referrer tracking than HitBox.
But this may be old info.. im not sure whether Web side story is still only exclusive HitBox as it was a year ago
Most of these poll results from sites without considering that a search engine got the visitor there in the first place around 90% of the time and the rest being repeat visitors who have bookmarked the site.