Well, that chart is only numbers, and there are quite a few variables at play to explain those percentages, most of them are pinpointed in the Ars' article linked at DownloadSquad.
It's well known, though, that nVidia had a rough time updating the ForceWare package of drivers to work with Vista and, at the same time, to properly support its new 8xxx generation of graphic cards (DX 10), which were released more or less at the same time than the new Windows. ATI didn't had those problems, as the current generation at the time was the Radeon 1xxx (DX 9), which already had its share of development by then, and the newer 2xxx parts (DX 10) were released in May of 2007. Maybe ATI adapted itself to Vista better than nVidia (in general, Catalyst got better under the umbrella of AMD than before), but this is something to be considered as well. Essentially, they got lost in the translation :)
Don't Apple use nVidia and ATi cards? Until recently they used nVidia 7300 and now they use ATi Radeo 2600HD XT or NVIDIA Quadro FX 5600.
Well, OS X uses OpenGL, and those cards are compliant with the latest spec, so there's no much hurry to use the latest and greatest card. Besides, I think the most graphic-hungry game available for the Mac is Enemy Territory: Quake Wars, which although a demanding game, it's not Crysis by any stretch. Still, the GeForce 8800 GTS is available at the Apple shop.