No idea, TBH, though I'd guess that the behaviour comes from the installed version of Windows Installer and the database schema comes from the authored version.

Nonetheless, if the solution is to add "ALLUSERS=1" to the command line when doing silent all-user installs, I'm okay with documenting that as being the fix for 2.7 and 3.4.

For Python 3.5, Windows Vista is the earliest supported platform, and so we can assume Windows Installer 4.0 or later (not that there's any need to take advantage of it) ref: http://msdn.microsoft.com/en-us/library/cc185688(v=vs.85).aspx