Ian,
the reason I found out X or Y ... not Z is:
I make a drill/mark on the table 0,0 (X,Y)
and I cut/mark the 0 depth.
after I cut "half sphere", I press home (0,0,0) and it shifted about 3mm from the mark I created before.
then I check the Z cut I created before, it is still at the same height.
I will take a photo tomorrow the shifted cut.