Indeed. It goes back to something that has puzzled me for a while. Why is there one more 150xx than there are 151xx. As it stands the daily requirements mean there are unused :-
2 x 150xx,
1 x 151xx,
2 x 152xx,
4 x 153xx.
Granted 4 spare 153xx looks a lot until you realise they need 4 or 5 on each 8-car set/unit.
A complete 9th set/unit will leave spare the following :-
150xx x 1,
151xx x 0,
152xx x 1 or 0,
153xx x 0 or -1(shortfall but see response by
@TimboM).
To my, crayonista, mind that means CS ordered the wrong number of coaches. It looks like 15011 (nightrider) should have been 15341 (sleeper standard)

.
Early on I thought the 150xx (nightrider ?) coaches must have something about them that makes them more of a maintenance overhead than the 151xx (sleeper lounge). Thus requiring an extra spare. Or maybe CS thought they might need to increase the number of nightrider coaches on, say, the Inverness portion. But that would lead to the Highlander and Lowlander having different formations. That would then not lend itself to the feature of the Highlander and Lowlander set pairs swapping each time they are at Wembley. Do they swap every day at Wembley or only every weekend btw ?.
I assume the swaps are done every day the sleepers are at Wembley because if it is every weekend then two pairs of sets(units) would never get swapped !.
Another thing I had forgotten about is that it appears the same two pairs of sets/units will be at Wembley every weekend. That therefore means that if Polmadie wants to do a coach/set swap on these two pairs they will have to be done during the day on Monday, Wednesday or Friday. The other two pairs would have swaps done during the day on Tuesday, Thursday or Saturday-Sunday. I am assuming that routine set/coach swaps will be done at the Scottish end as Wembley must be the furthest endpoint from Polmadie. I guess, if routine enough, that the ideal is when the coaches are on the Lowlander because Glasgow & Edinburgh are the nearest resting places - indeed they might well go to Polmadie anyway for cleaning etc ?.
But then I am assuming Polmadie (Glasgow) is the base for all these coaches - is it ?.