Introduction: How does the version numbering work?
See Wikipedia - Microsoft Version Numbering although they seem to have used shorter formats too:
Major/Minor Version Numbering
Installer Version Numbering
Software Version Numbering
A = Major Feature,
BB = Minor Feature,
CCCC = Major Build,
DDDD = Minor Build.
So, when major or minor features are added the whole version number will update.
All other less important changes will only update the build, which brings us to a rule for customers...
Check the major/minor version info, not the installer or software build!
Customers should check the version number, not the build number to determine if they have the latest.
For Windows XP, the latest version supported by the OS is 9.0c.
It doesn't matter if you take the offline or the web installer, they will result in the same version.
Then why are the build numbers different?
Because the installer has been rewritten because people where in need of an easier web install.
The build numbers differ by more than 1 because there were internal test versions in between.
The version shown in DirectX doesn't include the installer and thus differs from the one visible to you.
The offline version is 9.29.1962, the web version is 9.29.1973. Both are 9.0c.
It's reasonable to have around 4 alpha builds, 4 beta builds, 1 release build and the release itself.
In this small amount of installer builds, not much would have been changed.
This is why an equivalent version is detected, because DirectX itself hasn't been changed.