The DirectX End-User Runtimes Update includes optional components, which are
released in the bimonthly updates of DirectX SDK and includes files for
D3DX, XACT, XINPUT, and Managed DirectX, which are needed to run certain
game and 3D applications (this isn't unusual). The Runtime download won't
change the core DirectX components, such as Direct3D, DirectInput, or
DirectSound, since, it's not possible to downgrade DirectX, at least by
normal means.
To quote from
http://msdn2.microsoft.com/en-us/library/bb219742.aspx ,
"Generally speaking, the DirectX version number is not meaningful except as
a quick reference to the core runtime bits. This number should not be used
to check if the correct DirectX runtime is already installed, because it
doesn’t take into account the optional DirectX components". And, to quote
from
http://msdn2.microsoft.com/en-us/library/bb219721.aspx (Microsoft's
DirectX Frequently Asked Questions), "The name DirectX 10 is misleading in
that many technologies shipping in the DirectX SDK (XACT, XINPUT, D3DX) are
not encompassed by this version number. So, referring to the version number
of the DirectX runtime as a whole has lost much of its meaning, even for
9.0c. The DirectX Diagnostic Tool (DXdiag.exe) on Windows Vista does report
DirectX 10, but this really only refers to Direct3D 10."
The SDK is the DirectX Software Development Kit, for program developers. The
DirectX End-User Runtime is what you want. The DirectX End-User Runtime Web
Installer examines your system as to what components you don't have.
http://www.gamesforwindows.com/en-US/AboutGFW/Pages/DirectX10.aspx