I think the idea is that there is a difference between informing you of a one-way street, and telling you that you must go in a certain direction (which in this case would be along the one-way street).
For instance, this is the entrance to a one-way street. There are rectangular informational signs, because you need to know it's one way. But you don't have to go down it if you don't want to!
Further along the same street, there is a side road. Here you can see the circular "left-turn only" or "you must turn left" sign giving an order to traffic emerging from that side street. There is no other choice! Traffic already on Aberdeen Road can, of course, carry straight on or turn right.
Oddly, the rectangular informational one-way signs are not repeated immediately that side turning. Maybe that's a mistake, or maybe it's because all traffic has either just been told "turn right" from the side road or has already been informed of the one-way operation at the beginning of the street.
Similarly, this sign:
is interpreted slightly differently at junctions in different countries.