In NPC > Admin > Groups, the Properties tab has an option 'Include the children of managed items' .
Typically when creating a group of device it may or may not be desirable to have this option selected.
If for example the group is to be used for reporting on Availability and Reachability would not be selected.
However if the group is to be used for reporting LAN statistics then it would be selected.
In the case of a group consisting of just interfaces or poll instances such as for IPSLA or CBQoS there are no children to include and so should be left deselected.
On a side note, NV & eHealth will show a parent / child relationship with some interfaces embedded under another. This however is not applicable to this option in NPC.