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.
The BSDs got screwed over by a lawsuit in the 90s that made a lot of people hesitant to use them (coincidentally leading to the creation of the Linux kernel). Inertia carried it from there and Linux ended up getting more hardware and software support, which is the primary reason that people pick Linux over the BSDs now.