xCLV.GetBase.Top gives the Top propety of the xCustomListView object, which is the mBase object. xCLV.sv.Top gives you the Top propety of the internal ScrollView object which is a child object of mBase.
It's value is 0 because the top of the ScrollView object is on top of mBase.
So, you should use xCLV.GetBase.Top and not xCLV.sv.Top !