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This is talking about OSs not browsers

No. This is talking about Internet traffic by browser platform.

Macs accounted for 8.23 percent of all traffic on Web sites tracked by the firm during the month of September, up from 7.86 percent the month before. Those gains came largely at the expense of rival systems running flavors of the Windows operating system, which saw their combined share slip from 90.66 percent to 90.29 percent

http://www.appleinsider.com/article..._on_web_up_nearly_5_percent_in_september.html
 
Which sites did they track to make the statistics?

http://marketshare.hitslink.com/ said:
We use a unique methodology for collecting this data. We collect data from the browsers of site visitors to our exclusive on-demand network of live stats customers. The data is compiled from approximately 160 million visitors per month. The information published is an aggregate of the data from this network of hosted website statistics. The site unique visitor and referral information is summarized on a monthly basis.

Some specific customers they name are Forbes/WSJ/NYT/TheStreet.com/Computerworld/informationweek... (if I understand it correctly)
 
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