2. Left arrows show what the originating city receives from the trade route, and right arrows show what the destination city gets. Particularly for internal trade routes, the originating city does not always get more than the receiving city and they can, by happenstance, get the same amount.
This doesn't make much sense then. The numbers don't change on which city is originating, i.e. you would expect the values next to the left and right arrows to reverse but they don't.