The reason is that update.exe packages only install retail bits on retail builds and checked bits on checked builds. You can spoof Windows patches in order to install retail bits on checked builds, though, which is necessary in this case because checked bits of Windows Installer are not available. This does, however, require a registry change so I must give you the standard warning:
"Warning If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk."
Also, because you're installing retail bits the debug symbols for the previous checked bits installed with the operating system will be incorrect. The retail redistributable will not clean these up so install the retail redistributable on a checked build at your own risk. If you want the correct symbols, you might backup the previous symbol files and use the public symbol server to access stripped public symbols.
