Configuration Manager has (and has had for a very long time) the 'All Systems' collection. It contains discovered computers as well as actual clients. In ConfigMgr 2012, what you see in this collection (especially at 'child' primary sites) may be a bit unexpected.
For example, in ConfigMgr 2007, if you had a central ConfigMgr site (say site code CEN) and two primary sites (say site codes PR1 and PR2) and you viewed the 'All Systems' collection on PR1, all you see are the discovered objects and clients of that site. Same holds true for PR2, which may have been configured to discover a different AD domain, OU, whatever. Look at the 'All Systems' collection on the CEN site and would see all of the objects that PR1 and PR2 know about. But, PR1 would have no knowledge of the computers discovered on PR2, and vice versa. Makes sense.
Now, in Configuration Manager 2012, EVERY primary knows about ALL objects. PR1 knows about devices discovered on PR2, PR2 knows about devices discovered on PR1, and the CAS knows everthing too. Welcome to the world of DRS in ConfigMgr 2012. If you don't plan RBAC properly (or at all), this might pose some issues, especially around targeting baselines, packages, client agent settings, etc.