@davbram @midtsveen A market share is not always an indication of sales. It's also an indication of market competitiveness, i.e. how well a product is doing compared with other products of the same category.
Linux distributions are free but still compete with other computer operating systems.
@Sar Their FAQ says they use browser fingerprinting stats scraped from billions of Web sites. I wouldn't consider this accurate either.
For example, I've been a Linux desktop user for over 20 years but Statcounter would tally me as a Windows user because that's what my browser's user-agent says. I don't think it's uncommon for Linux users to spoof their user-agent header because there a lot of sites that block "unsupported" browsers.