I had a think about why it comes in so many pieces and have worked out why.
The screen and spoiler are two separate products - the screen is bike specific and the spoiler is generic so it makes sense for them to come separately.
With the spoiler, you have the choice of fitting it with one joint or two on each side and it comes assembled up to the point where you have to decide which setup you want. The spoiler had the first step of the instructions completed when it was delivered, which confused me for a while until I realised. When I was tidying up afterwards I discovered a slip of paper, which I had previously missed, that explained this and could have saved my confusion!
Andy