As it turns out, the Linux market share reported in the hardware survey for September was higher than initially reported.
Writing on Twitter, Valve's Pierre-Loup Griffais said this:
Just fixed a small rounding error in the HW survey that was affecting reported Linux usage. Distributions with less than a handful of users weren't getting factored into the total percentage. Numbers for September have just been updated: https://store.steampowered.com/hwsurvey/
With those fresh numbers, this means the Linux share for September was actually 0.78% not 0.71%. The recent figures now also show Windows dropping down to 96.29%. While that's a relatively small addition, it's still good to have it be more accurate.
I've updated our dedicated page tracking it, which shows that Linux is now hitting a 16 month high in terms of Steam market share. I don't think the relevance of that can be overstated, considering another 0.15% would put us back to a level we were at in September of 2016. Although, we would have a lot more users considering the ever-increasing growth of Steam.
One thing to be clear on though, is that we have no idea how long this error has been around. Meaning it could have affected months or years worth of data.
This isn't the first time there's been a flaw in the survey, like the time Valve was essentially over-counting cyber cafe customers.