I do love how broken the FB engine is :S
That said I suspect a lot of the difference is due to the level that nVidia has hacked DX11 functionality and hand optimised replacement functions for code that sits between the DX core and the drivers - both for threaded performance and for better compatibility with nVidia GPUs. In some games its not so significant but it seems in BF4 and by extension hardline that it has quite an impact there.
That said I suspect a lot of the difference is due to the level that nVidia has hacked DX11 functionality and hand optimised replacement functions for code that sits between the DX core and the drivers - both for threaded performance and for better compatibility with nVidia GPUs. In some games its not so significant but it seems in BF4 and by extension hardline that it has quite an impact there.