DirectX 11 magically gave 10 cards new features.
Technically the WDM 1.2 and 1.3 changes gave the DX10 cards new features and performance. DirectX's use of the WDM/WDDM was then able to take advantage of some of these advancements.
Even the original Xbox 360 GPU isn't technically DX10 capable, but its DX superset if somewhere between DX10 and DX11 depending on the features.
Direct3D is very closely tied to the OS video model technologies, unlike OpenGL, so advances made at the OS level do improve functionality beyond what the original hardware was designed to support.