I believe OS X has graphics drivers provided by Apple, not by the hardware vendors, so that likely has something to do with it.
Are you sure? I'd think it isn't passing function calls, but rather direct hardware access. I might be wrong.
Either way, there's clearly a way to make the game run significantly faster on OSX, but Valve isn't doing it for some reason.
They use the platform graphics APIs like any other app.
Parallels probably work by decompiling DirectX shader bytecode back into GLSL, which is already optimized for OS X shader compiler, therefore there is less burden on the optimizer.
Another example of poor drivers are various GLES drivers on mobile, mostly trivial optimizations can drastically increase the performance: http://aras-p.info/blog/2010/09/29/glsl-optimizer/ and https://github.com/aras-p/glsl-optimizer