Err, just to make sure that my reply did not confuse you:
Its not that you dont get physixs with an ATI card... (they do it without the need for extra drivers, or extra settings)
Its that Nvidia "supposedly does it better" since they have all that extra driver crap for it.
Whether or not its actually better...
Both card types make the game LOOK the same... however since the physics calculations are optimized for Nvidia cards, they typically get + 10-20% better framerate over ATI cards in those games only.