I'm not sure how many ARE common ...  Even starting in coach A, there are differences ...

 

Although it doesn't really explain why the software isn't programmed to allocate coach/seat numbers that are common to both configurations and leave the seat layout that are different as the unreserved seats.

Also not clear why reservations are binary, reservations or none, as in has to be whole train that becomes unreserved, rather than just part with different configuration.

At least get reservations on some services, Portsmouth-Cardiff trains are still fitted (and presumably will still be in 2030 and beyond), slots in top of seat backs for card reservations, but no retrofit electronic reservation displays.

 
That and of course it is a coach shorter and there is no coach E on a nine car, so anyone with seats there (admittedly first class) would have to be re-directed to a different coach letter.

 

I think it's because the standard / first varies in which carriages its in between the five and nine car sets, so it also varies the numbers of seats and layout.

 
10:03 London Paddington to Penzance due 15:03
Facilities on the 10:03 London Paddington to Penzance due 15:03.
Will be formed of 10 coaches instead of 9. There are no reservations on this service.
And the return trip from Penzance
16:15 Penzance to London Paddington due 21:24
Facilities on the 16:15 Penzance to London Paddington due 21:24.
Will be formed of 10 coaches instead of 9. There are no reservations on this service.

Also on
13:15 Plymouth to London Paddington due 16:29
Facilities on the 13:15 Plymouth to London Paddington due 16:29.
Will be formed of 9 coaches instead of 10. There are no reservations on this service.
And the return trip from London
17:03 London Paddington to Penzance due 22:24
Facilities on the 17:03 London Paddington to Penzance due 22:24.
Will be formed of 9 coaches instead of 10. There are no reservations on this service.

Eh?
Reservations not transferrable (software issues?)?