Assuming the installer is used the application must provide it with information that is unique to each and every install such as where the various files go, what permissions to assign to each, etc. This information ends up in /Library/Receipts as a .pkg file. These files are not only used to direct the actions of the Installer, they are also what Disk Utility uses when it performs Permission Repair. Compatibility checks would be part of the instructions provided in the installation package.
I suspect Alan's download did not use an installer and was simply a drag and drop installation in which case there would be no compatibility check until he attempted to run Safari and the OS recognized the incompatibility. Even if the installer was used, it is entirely possible the compatibility check was omitted from the pkg by oversight, by lack of knowledge, or even by intent such as also installing additional items such as hooks that can be used subsequently to remotely install malware. More than one trojan has used that ploy.
Alan was probably using the third party sight because what he was attempting to do is not supported by Apple.
