Having an explicit ninth set does seem inconsistent with the way things are operating at the moment, particularly with the Fort William 'day coaches' and the way in which they get swapped over since there are only 10 Club Car vehicles available.
It has been puzzling me a bit (just looking for something tidy) and they don't seem to need the ninth set.
To recap there are supposed to be nine 8-coach sets with eight in use on any one day. - 72 coaches.
Also a seated coach (150xx) and a club car (151xx) to add to the FortWilliam portion at Edinburgh. - 2 coaches.
Finally s seated coach (150xx) that is totally spare - 1 coach. Now, as explained up thread, that is an important spare as an 8-car set (or shorter) cannot operate without a sesated coach. It has the brains in it to control the doors, brakes and maybe other things in upto 8 coaches. So there is one more than the club cars.
So a total of 75 coaches.
Now with the 8-car sets there are two formations as four of the eight in service have a normal sleeper coach (153xx) replaced by a PRM version (152xx). The ninth 8-car set is the same so it is not a perfect replacement for 50% of the 8-car sets in service on any one day. To me that undermines the point (if any) of having a complete 8-car set spare !.
As you say. They don't seem to need the ninth set in its entirety. Mind you we don't know how much the fitters at Polmadie are pulling their hair out coping. Possibly with overtime in which case it is the management pulling their own hair out !.