Isso se deve à nova tecnologia Click To Run do MS Office. Veja o que MS tem que dizer :
Assume that in a Click-to-Run edition of a Microsoft Office application you run an Office add-in that creates custom registry entries under the HKEY_LOCAL_MACHINE (HKLM) hive. In this situation, the custom registry entries are created in a specific part of the Click-to-Run registry subkey. For example, if an add-in creates a registry entry under the HKLM/Software hive, the following registry entry is created:
HKLM\SOFTWARE\Microsoft\Office\ClickToRun\REGISTRY\MACHINE\Software\Wow6432Node\Custrom-Registry-Entry
This behavior is by design because Office Click-to-Run editions use Application Virtualization. Any custom registry entry under the Click-to-Run hive may be changed by any Office update because the Office Click-to-Run editions own the Click-to-Run registry entries. For example, any custom registry entry may be deleted after you install an Office update.