The problem is that not all sites see the same kind of traffic. These figures are from NetMarketshare which gets most of their data from a modest number of large corporate sites[1] and uses their estimates to adjust traffic country by country. Their numbers skew heavily towards IE, so I'm assuming that their sites must see a lot of business users. If you run a site which was aimed at home users you'd see a much great representation for WebKit and I'm sure sites like HN, ArsTechnica, etc. have a higher percentage of the latest browsers.

I have never worked on a site where the breakdown is close to NetMarketshare's figures. The ones from Akamai have been much closer, particularly internationally:

http://www.akamai.com/html/io/io_dataset.html

1. http://www.netmarketshare.com/faq.aspx#Methodology