It peaked at 4.05% in March. The last 2 months it went just below 4% as the Unknown category increased. For June the reverse happened, so 4.04% seems to be the real current share of Linux on Desktop as desktop clients were read properly/werent spoofed.
Because it’s not overly popular as a desktop os, you are far more likely to see it in certain appliances and server applications etc, none of which will show up in a pagevisits based statistic.