It's actually some combination of Apple and NVIDIA drivers, but on OS X, unlike pretty much every other OS, Apple insists on owning the OpenGL entry points and doing a bunch of work in its own driver before calling down into the vendor driver's OpenGL entry points. The vendor driver then has to undo some of the "helpful" things that Apple did, and work around whatever bugs are in the Apple code. For NVIDIA, in particular, the OpenGL driver is pretty much the same between Windows and Linux, and so one shouldn't be much more broken than the other, at least when it comes to core OpenGL functionality. But the OS X driver can't take as much advantage of the common code because of the Apple code that it has to deal with.