Adding smart groups together

On my databases I have a number of groups with sub-groups.
When the sub-groups are normal groups the main group give the total of all documents in the main group and sub-groups.
When the sub-groups are smart groups their content in not included in the total for the main group.
Have I missed a setting, or is this the way DEVONthink works, as it is a little frustrating.

Smart groups hold no document content. They are search queries that activate when opened.