Copper/Ground Fill only on ONE side?

I just did some more experimenting and found this.
If you have a 2 sided board, and on the bottom toolbar both layers are selected.
The 'routing/ground fill' button will show both top and bottom ground and copper fills.
If you select on the toolbar which layer you want to fill (top or bottom),
then the 'routing/ground fill' button will change to show only the selected layer to fill.