The original DfT plan had splitting and joining at other locations, such as Swindon and Cardiff, so by operating all the sets to Swansea as 9 or 10 car, you are not seeing the original plan in action, which also didn’t have any extra trains west of Swansea.
The Swansea services should be, in the main, 9 car cl.800 sets with a minority as 10 car cl.800. GWR changed some of the stock workings at the December timetable change date but I suspect any variation you saw in the previous two months had more to do with availability on the day.