When discovery is performed by subnets, a full audit is performed on each discovered device where a login can be accomplished by the supplied credentials. Often this is a bunch of Windows machines and audits retrieve lots of information.
When discovery is performed by Active Directory, it seems that such full audits are not automatically performed? My experiment retrieved a basic summary of each client computer but not partitions, installed software, etc. Of course details would not be retrieved from machines which aren't joined to the domain, but the domain administrator's credentials should be able to log in to every computer in the domain. I wonder why Active Directory discovery didn't perform full audits on domain members?
Make sure you're not running "Import AD". This will do as you describe. Otherwise, yes, a full audit should be performed when AD is discovered.
I haven't done anything that would run "import ad" but what does "import ad" mean? I don't see any settings with a name like that. Google finds some unrelated sites with that phrase, but not here yet. (When Google indexes this page, this will likely be the only match on this site.)