Been having a gander through the new timetable...
1722 Portsmouth to Cardiff Central in the current timetable will be cut back to Bristol Temple Meads, and an additional 1959 Bristol Temple Meads to Cardiff will operate approximately 5 minutes later throughout calling at Filton Abbey Wood, Newport and Cardiff Central.
Odd.
That's a 5 minute later departure, compared to the current timetable, from Bristol Temple Meads.
I share your curiosity on this one.