From BlenderWiki
Bounding Box Center as Pivot
Mode: Object mode and Edit mode
Hotkey: ,
The bounding box is a rectangular box that is wrapped as tightly as possible around the selection. It is oriented parallel to the world axes. In this mode the pivot point lies at the center of the bounding box.
In Object mode
In Object mode, the bounding box is wrapped around the object centers and does not take into account the ObData (geometry).
In Edit mode
This time it is the ObData that is enclosed in the box because all of its vertices were selected. The bounding box in Edit mode takes no account of the objects centers but only of the selected vertices.
