Niall,
Where's Melyvn when you need him?
I've read (probably here) that Old Tom wanted Muirfield to open the 16 holes that he had ready to play, and that he built/opened the remaining two within a year.
Old Moray's final two were constructed only after the club leased the land they needed to build them, so OT was either happy to go with 16 or he knew that the extra land was going to be had for the remaining two.