Yeah there’s no way I trust their methodology has stayed that stable over 15 years.

There's likely some decently sized error bars. I'm not sure how accurate Statcounter is. It would be nice to see data independently acquired by another service to increase the confidence level. If Statcounter is only checking the useragent, then the accuracy of the data is certainly lacking, but the trend is worthy of note, or, at least, further investigation.