I think you’re getting confused by the terminology; this is an explanation from the Wikipedia article.
The terms right- and left-hand drive refer to the position of the driver and the steering wheel in the vehicle and are, in automobiles, the reverse of the terms right- and left-hand traffic.
ETA: So the red areas have left-hand drive and right-hand traffic, while the blue areas have right-hand drive and left-hand traffic.
That doesn’t explain why they have the cars meant for side A, but use them to drive on side B.
cars that drive on the right have their steering wheel on the left side (i.e. left-hand drive), and vice versa.
OP said (and stressed with italics) that they have right hand drive cars but drive them on the opposite side, which was the point of his comment
I think you’re getting confused by the terminology; this is an explanation from the Wikipedia article.
ETA: So the red areas have left-hand drive and right-hand traffic, while the blue areas have right-hand drive and left-hand traffic.