I suspect that as the production of these trailers is a very manual process, the expansion of their facilities allows ETI to double the number of trailers they are working on at any time rather than reduce the time taken to build one by half. However, the expansion does allow an increase in efficiency which will naturally lead to a reduction in building time.
That being said, I think that ETI are being extremely cautious about managing customers' expectations. Building in a little slack time for them to work with allows them to ensure that all their production is completed as promised, even if they encounter any production issues which will inevitably occur, its just normal - its going to happen. Way better to under promise and over deliver, in that way you look like a hero if things are early, rather than over promise and under deliver, where you look like a failure. Has anyone on the forum ever reported that ETI didn't meet a promised date?