By default the origin of the spread is to the upper left. This makes symmetrical placement with exact numbers quite awkward as you have to manually add and subtract the page width. There is already a setting for using the spine as origin, but in my view this is the sensible default value. I can't really think of any reason why you'd want to define coordinates on the right page from the outer corner of the left page. An origin that lies symmetrically where the pages meet makes so much more sense to me.