Its based on the serviceability of the various navaids (not the ILS obviously). If you look at the missed approach procedures, they are based on different VORs (normally) to cater for unserviceabilties. I can't be sure (it's been awhile since I have been there) but I think the Z approach is for fully serviceable navaids, enabling more than one VOR to be used for the missed approach.
I'm prepared to be corrected!
