Going back to basics, when the order was placed Wales rail usage levels were different, there was no pay as you go in South Wales, some lines were deemed 1 for 1 replacement, so a 2car 150 became 2car 197 etc.
Was almost done on a no growth basis at the time. Strictly the capacities changed a bit, but wasn't really provision for needing longer trains on majority of network.
197 fleet
is 30 x 2 car (no ECTS)
and 21 x 2 car with ETCS
and 12 x 3car (basically a 2car with centre vehicle)
and 14 x 3car partly with better seats for higher than standard class section
So 4 relatively small sub fleets, but configured in such a way as can't run 3car (on a service that is too busy for 2car) on ECTS equipped Cambrian, it's 2 or 4 (or 6 etc), but not 3car
Assuming the trains with better seats were intended for longer routes (even if not yet used that way), then isn't really much flexibility in standard units either (30 2car but just 12 3car), again if a service is too busy for a 2car it's going to be very clunky trying to lengthen it.
With so few standard 3car units, if swap in a 3car, probably forced to steal from a route that really needed at least 3cars, so do you change that to a 2car (with potential overcrowding), or use 2x2car (and doesn't seem to big enough fleet to run pairs of standard 2car units normally, seeing as many of the smaller branches need the 2cars)
So altering the mix to fit late 2020s traffic levels with growth seems a reasonable option, otherwise basically stuck with no growth levels when fleet was planned decade ago.