I think its because the offset is the centre of the selected not just the turntable.
to rotate everything around a specific point i would do either
select the turntable only, <SHIFT-S>, Cursor to selected. Then change the rotation to rotate around the cursor, with <.> this will give everything the same point of rotation
Or you could parent every object to the turntable with CTRL-P (make sure the turntable is the last one selected to make it the parent) then when you rotate just the turntable everything will rotate as well.
Or you could use constraints i think such as the copy rotation constraint. not sure bout this one
Anyway this should help, let me know if i havent been clear or if you need something different