http://en.wikipedia.org/wiki/Usage_share_of_web_browsers#Dif...
You can click around on each source, and they have an explanation of how each source get their numbers and adjust them. It's very insightful to understand the differences in measurement. For example:
> StatCounter statistics are directly derived from hits (not unique visitors) from 3 million sites using StatCounter totaling more than 15 billion hits per month.[17] No weightings are used.
> Net Applications bases its usage share on statistics from 40,000 websites having around 160 million unique visitors per month. The mean site has 1300 unique visitors per day. On 1 August 2009, Net Applications began weighting its raw data based on the number of internet users in the countries concerned, using data from the CIA ; the changes were applied retroactively to older data.
Apples and oranges. Net Applications measures users, while StatCounter measures usage. It is entirely possible both are right, and the different numbers are both accurate about what they measure.
It is also possible one or both is wrong, possibly by a large amount - we don't have any good data against which to validate them (that data exists, but it isn't public - Google and Facebook know the truth).
I had to make a website for a brand with strong sales in China. The client asked for a full IE6 support "because chinese use IE6 according to X and Y." IE6 support is expensive.
I was quite doubtful, the brand is a luxury one, kinda confidential, so the user had to belong to the new rich class which can afford a real computer. But they did not have any data and did not want me to gather data on their actual website to confirm or infirm my hypothesis.
The IE6 friendly website lauched. The stats reports less than 4% IE7 and less usage (including funny browser derived from IE.)
"IE6 somehow managed to gain 0.06 percentage points", which may indicate the NetApps data isn't very reliable. (maybe?)
Browser market share is so incredibly off. I know of 50000 IE7 users who don't even put their browser near the internet.