The idea was to have it mechanically different from the other screens.
If it was doing per-second consumption, there would be no difference between this, and simply adding another produciton building. Whether it's a good implementation or not is up to the debate, but I thought that it should be at least different.
On top of it like this, it's the only place where the size of stockpile actually matters, since for some contracts you would actually need to bump up the production to reach the required stockpile size to initiate the trade in the first place (as you mentioned in your post). So it adds a bit of diversity to the routine.