OpenGL is a bit more complicated since it’s more than just a specification in practical terms. The documentation and tooling for OpenGL was really awful compared to Direct3D. This is a huge issue when developers are working on implementing features. For instance, the documentation for glReadPixels is incorrect for years and you would have to refer to the wiki for it instead. Yet, the only way you would know this is if you scoured the internet and happened to find a StackOverflow page asking about symptoms that may not even match your issue.
Thankfully, Vulkan seems to be a lot better in this regard but I still curse the heavens everytime I need to go back to OpenGL when supporting older hardware.
Not really a surprise considering the size of their domestic market. An interesting statistic would be each of these vendor’s shares outside of their home country. I’m particularly curious at Apple’s share without the US’s iPhone majority.