Fair enough, obviously an error with the reservations there then as C was heavily reserved. It’s unusual for that to happen - usually the TM‡/driver can correct it if the reservations have loaded (which they must have done for the green lights to have been on) but the formation wrong, unless the previous train had stuck in the system? Did you notice whether the internal/external displays were showing the correct information, or what reservation information was being displayed in other carriages?
Reservations are generally much better now, but still not as reliable as they should be.
External carriage displays that I observed were correct. The internal display in my carriage clearly showed "C". Don't know about reservations in other carriages although another passenger stated that there were no reservations on the train so there may have been an announcement before Chippenham. Certainly nothing afterwards.