Just because its running as a 2 car, does not mean there is a 2 car 165 set down (well there is now). However before the incident, they were a 3 car 166 down, which became a 2 car. This 2 car 166 is then pooled with the remaining 2 car 165s for diagram allocation purposes, but it's never as straight forward as that.
At the moment, GWR have 1 x 166 away for overhaul, 1 x 166 re-formed as a 2 car and added to the 165 pool, and a 165 down due to damage.
So apart from scheduled maintenance, by reforming a set, GWR are only 2 x 3 car sets down which would otherwise be available on a daily basis. I should add that they are a 166 down anyway longer term, and this is factored in, so in net effect, they are a 3 car set down, where last week, they were just 1 carriage down.
Depending on availability, various different diagrams are short formed in order to provide the best use of available stock at the appropriate times and in the right places.
For example, 6 car sets becoming 5 cars, and 5 car sets becoming 4 or 3 cars etc etc. Even one set out can cause many more knock on short forms.
Those who work with stock allocation maybe able to explain further