The same wikipedia page gives the methodology applied for those calculations.

- StatsCounter (hits, not unique visitors)

- W3Counter (last 15,000 views from 50,000 websites)

- Net Applications (stats from 40k websites having 160 million unique visitors per month)

- Wikipedia (page requests)

Only NetApplications is using unique visitor stats, the rest are tracking views. The results are in line with what most people would assume; Chrome and Firefox users are more active users. But in terms of number of unique visitors IE is still far ahead.