Presumably, one is supposed to be a 3-car, but the 2+2 coming north is all they had today.
It does not help that the 20:30 would otherwise be a Mk4. I can see why they do not send the Mk4 back to Cardiff though. If Crewe was not so busy the 19:30 could be 4-car to Crewe and leave a 2-car there. The Mk4 set run in service to Crewe as the 20:30. The 197 dropped at Crewe could then be the 21:11* from Crewe. That would probably clear most of the capacity issues from Manchester, but creates a lot of messing about. The current situation is the most effective, even if there is not as much capacity leaving Piccadilly as there could be.
In a parallel universe, Northern could provide a Crewe service to give more capacity. In a distant parallel universe there would be crew to run the Mk4 in service back to Crewe (presumably needs a spare guard, since TfW must have a spare driver at Picccadilly to work the 20:30 after the driver who works the Mk4 in from Cardiff takes the Mk4 back out ECS)