Yes and No. A majority of the games on the Mac are ports from The PC which were created for use with DirectX. Companies like Aspyr port games and have to convert most of the games to OpenGL. Plus other data. Games such as WoW normally have pretty good performance on the mac because they were built side by side with the PC version etc.
I think that's what it is anyway, I remember seeing something ages ago on here about it, do a search and ya may find something 🙂