Browsers extensions and Defense+


I noticed when downloading extensions for Dragon or Opera that I receive just a single alert from Defense+ (paranoid mode) telling that Dragon.exe or Opera.exe is trying to access explorer.exe in memory. The extension is installed without more notification.
When sth is installed trough IE, defense+ alerts more.
Do the install silence Defense+ because they become part of the browser or what else could triggered this lack of reaction of Defense+?
Should it be possible for a malware to simulate an extension and be installed silently?