The short formations go against their reliability scores which are already terrible. So there must be something fundamental driving the short formations or just reclutance that they won't be penalised.
Ultimately, GWR▸ will make the decision on short forms with the daily allocations. For example, if a set is going to make a depot - Paddington - Bristol - Paddington - depot working, that’ll be prime candidate for a short form. They aren’t going to hold 2 5-car IETs▸ back just for that working.
Basically allocations will be done to maximise the number of seats available throughout the day with emphasis on the key busy peak services.